The commands to undo and redo changes are:
You can also move back/forward in the history of changes. For example, to set the state of the file as it was one hour ago or how it was one hour after, use:
:earlier 1h :later 1h
Time can be specified with a number and s, m, h or d for seconds, minutes, hours or days respectively.
Vim has a very complete undo support with recording of all undo branches and undo persistence between sessions. Check :help undo to know more about these advanced features.