Once the cylinder is out of the lock, you can take a small screwdriver and push the tumbler out from the bottom. The spring MIGHT eject, so although this sounds stupid, I wrap cellophane tape around the tumbler tops allowing enough "give" to push them out a bit. Keeps them from flying out and the spring going to somewhere you won't see it. It takes a little bit of pressure, but once you do one, you'll quickly get the hang of it. To replace them whenever or if you want to, you just place the spring in the hole, then "snap" in the tumbler that came out of that slot. Hence the name "snap in" tumblers.
Thing is, if you want to ensure they match the key, you'll need to keep them in the proper order as they come out, front to back. Some have a number on them for depth markings. Some may not. Factory ones normally do, but not always. Note too, those sort of locks only used the last 4 key cuts for the tumblers. First two aren't needed.