How to revert (undo) a change and how to revert the revert (redo) in Vim?

It's very simple.

1. Make sure, you're in NORMAL mode by pressing Esc just in case.
2. Press u to undo the last change. You can prepend it with a number, like 5u, to undo the five recent changes.
3. Press CTRL-R to redo the change that was just reverted. Again, prepend it with a number to repeat the operation multiple times.

That's what you need to know to get going.

I hope that was helpful.

What's next?

• To learn more (there's so much more to it!) about those operations, please refer to the help (:help undo).
• Install the undotree to never loose your changes again!
• My quick list of the essential Vim plugins for starters

Have a productive day!