What resources do you recommend for an adult wishing to relearn math?

Kahn academy. It’s free and goes as deep as you want. I had to brush up on some stuff and it was great.

Khaaaaaan!
Some men just want to watch the world learn.
If I ever won the lotto, I'd donate a big chunk to Sal. He got me through my worst classes. Him and the organic chemistry tutor on youtube, who also does lots of easy to follow math.
Khan academy got me through the end of high school and engineering. It really made the concepts a lot more understandable than the lecturers.

If it's content is still up to scratch, I hope it's getting the recognition it deserves!

https://www.khanacademy.org/

Helped me get through my engineering degree. Absolutely the best maths education I've ever seen.

Started!
- You got this!

As others said, Khan academy, but in the event that you need something even more broken down, patrickJMT on YouTube is a godsend.

That's a name I haven't heard in a long time.

I used Khan Academy when I reentered uni as a mature-age student and found it very helpful

On a related note, what math should one know? Are there any upsides to go beyond everyday math? To brush up on lost math skills? I've forgotten most of my math classes, as I wager most have...

Math is one of those funny things that's always all around you even if you're ignorant of it. The ancient Greeks calculated the circumference of the world to an impressive degree of accuracy and they didn't even have calculators - they just looked at some triangles and made some guesses.

Do you need to know the circumference of the world in your day-to-day life? Probably not. But it's cool as heck knowing that you can figure that out by applying the right formulae.

If you know math and you're faced with a problem that can be solved with math, then you can solve the problem. If you don't know math and are faced with the same problem, you might not know how or if it can be solved.

Your specific question: "Are there any upsides to go beyond everyday math?" is an interesting question because of the implication of what 'everyday math' is. Depending on our professions or interests, your definition of everyday math might be radically different from someone else. Numeracy is enough to go on for a lot of people, which often implies arithmetic. But hey, fractions are always coming up in places, and if you add algebra to the mix you can start solving some interesting problems.

Some level of applied mathematics are used in all sorts of fields. Construction, Finance, Medicine, Software, Logistics, Conservation, Cooking, you name it. And the beautiful thing about a lot of these cases is that you don't need to know very complex math in order to follow along an established procedure. For instance, I don't need to know how to find the proof for compound interest, but I can easily look up a formula so I can make some projections of my investment plan.

Anyway, long story, but math is one of those 'use it or lose it' things. And if you've lost the math, then you start to be unable to see where math can be used. Keeping the math alive or rekindling it opens you up to possibilities that you might otherwise be ignorant of. Learning math for its own sake is fine, but finding ways to use the math you've learned is what helps keep it alive, and broadens your own horizons too.
- I feel like I'm haunted by linear algebra because it keeps cropping up in all sorts of places
I make my living doing pretty basic math that people are too lazy to learn or too afraid of. Financial simulations and shit like that. Pays to understand at least the most basic probability, statistics, calculus. I used to rely quite a bit on dynamical systems theory and linear algebra, but that was years ago now. To be fair, you also need to learn to code this shit up, but that's not hard, either.
- Well, I am about to move into a business analyst role, so I’m figuring maybe it can’t hurt for that either.
I feel like there are some interesting ideas in pure math topics like Real Analysis, Abstract Algebra, etc. Although I'm terrible at actually writing proofs and such.
- Formalizing e.g. limit is quite interesting! Limit is related to tendency; sequence x_n converging to x means for large enough n, x_n is sufficiently close to x. That is, you can choose N such that | x_n - x | < eps for n >= N. In some sense, you are concretely defining what rough terms mean!
  
  To look into these, you can read through books disregarding proofs. While proofs do hold ideas, they can be headache-inducing.
Personally speaking, I absolutely suck at math. It was and continues to be my worst subject. Likely to do with my adhd.

I was only able to really get up to and through basic algebra and some geometry in school. Past that, nothing else. I do fine. I'd say thats the minimum unless your in a field that requires a higher level of math.
For most people the basics is enough. If you want to change careers to be an engineer or scientist or something like that you would want to relearn math

The best source I know: https://betterexplained.com/

Also plenty of youtube channels, like Numberphile (many of the featured hosts have their own channels), 3Blue1Brown, Mathologer, Wrath of Math and many more. They have vast libraries covering pretty much any topic imaginable. It's all top tier presentation, so intersting they made me study math for fun - I'd rather watch Numberphile than Netflix.

Brady Haran was a journalist and is is excellent at explaining things

You have to write out a lot of exercises and there is no getting around it. You can't learn the violin by watching videos or reading a book. You have to practice. It's the same with math. But as people said, Khan Academy lectures are very good in steering you through a topic.

Besides algebra, I think it is important to know a bit about probability and a bit about logic. Don't worry about stuff like covariance matrices, but understand what conditional probability is (be able to explain the "prosecutor's fallacy") and write out some of those annoying exercises about urns full of colored balls. Also, show how to write e.g. "you can fool all of the people some of the time, and some of the people all of the time, but you can't fool all of the people all of the time" in predicate logic notation, and see how the parts of the sentence involve switching the order of quantifiers.

Another comment mentioned Baron's workbooks. Any other resources for exercises which you'd recommend?
- I'd expect textbooks would have tons of exercises at that level. Schaum's outlines are good for college level math but I don't know if they have them for stuff like basic algebra. I have a friend who is a HS math teacher so I can ask her for recommendations and get back in a day or so, hmm.
- Ok, I emailed my friend (above) and she said Khan Academy and she says it has exercises. That's great, I had thought it was just video lectures. So I'd go for that.

As others have said Khan Academy has helps plenty of students so I'll recommend another yt channel.

Has has multiple channels but this particular one, bprp math basics, goes over tricky math problems students would solve and he goes over his solutions step by step fairly well

In terms of british highschool level

I got 345 videos from a maths watch DVD I hold dear to

Just reply with 'yes' if you would like that

'yes'
- Yes

Professor Leonard. Check the playlists https://youtube.com/@professorleonard

I cannot recommend english-speaking content but if you happened to speak french, as well as for all french speaker on this post : The king of all math teacher Yvan Monka has anything you need on YouYube from middle to high school @ymonka@youtube.com
And to learn to love math and interesting facts about it look for Mickaël Launay @micmaths@youtube.com

Grab a test prep study guide - GED, SAT, ... You can probably get a super cheap one at a used book store.

If you graduate to college level you can try Opencourseware -> https://en.m.wikipedia.org/wiki/OpenCourseWare

Take some basic logic classes first. It's really helpful for a lot of people before learning math and science. I didn't realize how many people aren't just logical thinkers by default sine I am. But being able to consciously think that way will help a ton with math.

For relearning all school-level maths and terminology,

mathsisfun.com

has very concise explanations of maths concepts.

I really liked the "Baron's" brand workbooks. I re-did some high school math with those. They explained the concepts and also there were many exercises to do by hand.

I used www.purplemath.com when I had to pass math test to teach. I passed first time.

In the past I wanted to do the same because I have the same problem as you but I never actually gotten around to executing the learning part... The one resource that picked my attention the most is https://youtu.be/didXE0HkSC8 ("Learn Mathematics from START to FINISH (2nd Edition) "). It's a 37 minute video with dozens of book recommendations and how you should proceed with the order of those books.

I haven't looked for math classes but I just found classcentral.com last night. They have an unbelievable number of free classes, like tens of thousands. Seems geared to earning actual certificates etc. but I found tons of computer classes and the one I focused on and watched several chapters was excellent. Very clear and easy to follow. Seems a little hard to find anything TBH - any search returns a flood. But who knows, worth a look.

I've had good luck on a number of subjects by getting DVDs produced by The Teaching Company from my city library.

I haven't looked for math stuff yet, so not sure what they have.

https://www.youtube.com/channel/UCqu9Na4YBSijiwdDj1KpyRg