So maybe you have to change your bearings, or eventually you will have to clean them. Now, you can get a bearing press to take out your bearings and put them back in. It's a lot easier and a bearing press isn't that expensive, like maybe forty bucks? But I know how to do it without, so I've never bothered. It's worth getting a bearing press, though, if you have wheels with metal hubs, which are practically impossible to change by hand. Which is partly why I gave up on metal hubs.
To take the inside bearing out, you pry it off with the axel. I'm not going to be able to describe exactly how in words, and the picture isn't exactly right either. You put the wheel on the axel straighter than in the picture, tilting it just slightly so the threads catch the bearing; then you yank the bearing straight out. Fiddle around, you'll get it.
To take the outside bearing out, you use the axel again to pop it out. But you don't want to damage the shield—that rubber ring thingy— or the bearings behind the shield. Luckily, the government manufactures metal discs in various sizes that you can use to cover the bearing for protection, called "pennies" and "nickels." Put a penny behind the outside bearing...
...then flip the wheel on the axel. So the penny is in contact with the axel, right? Give it a short, sharp strike with the flat of your hand, that should pop the bearing right out.
To put the inside bearing in, just press it into place with your thumbs. If you have one-sided bearings, the shield side faces out and the open side faces in. If you have two-sided bearings, it doesn't matter.
You will need a coin again to pop in the outside bearing. This time, put a nickel on top of the outside bearing.
Then I put my skate tool on top of the nickel and give it a short, sharp strike with the flat of my hand.
