If you live in Denver, you should come down and let me change your strings for you. It'll cost you a few extra bucks over doing it yourself, but holy cow it'll save you a lot of time and be way more fun. We always have a pleasant conversation and you learn to do this in a live session. But, if you live outside of Denver, or are a wacko about wanting to do everything yourself, then hopefully this helps.
You should be able to change strings in about 15 minutes. Hopefully this video will be a shorthand help in making that happen. You need to change strings once every year no matter how much you play and for those of you who play daily, you probably need to change them more often. Playing ukulele is challenging enough, so unless you like sounding iffy, keep fresh strings on your machine.
If your strings are white, black, or if you never need to tune up your ukulele, then you could be sounding a lot better with a fresh set of modern strings. Be aware these "best strings ever" things you read on the internet are mostly non-sense. There's only a few string manufacturers and everybody buys in bulk from them and then cuts them up and puts 'em in a fancy package. I've tried more or less everything on the market and the sales pitches are beyond ridiculous, but they probably work. Just because somebody you like says they play a certain strings, doesn't mean it's true or right for you.
After trying a bazillion strings over the last 25 years, I keep it simple and recommend fluorocarbon ukulele strings for all ukuleles.
For High G ukulele: I like the Savarez tenor ukulele strings (Savarez Ukulele Strings Alliance 150R Tenor) because they're readily available and will fit all ukuleles and the price makes sense. There's no need to buy concert or soprano size. Just get the longer ones as they're faster and easier to put onto your ukulele.
For baritone and Low G with wound metal strings: I like Savarez classical guitar strings (Savarez 540R NT Alliance).
Low G Notes: Do NOT put wound metal strings onto your regular ukulele unless it is built for them. I've ruined three of my own ukuleles over the years trying to get fancy. If you want to try Low G on a regular ukulele, you can put a plastic Low G string on it, but be prepared for it to sound horrible. If your ukulele is built for a real metal wound Low G string, you should use strings 1 thru 4 from the guitar set and throw strings 5 and 6 away.
Baritone Notes: For most baritones, I use strings 1, 2, 4 and 5 from the guitar set. Some players who do a lot of heavy strumming might prefer a 2, 3, 4, 5 set-up, but I've never found the trade-off to be worth the effort.
Let me know if this is helpful to you. If something isn't clear, please post a note in the comments section of the video and I will either edit the video or make a new one if necessary.