Apple GarageBand 2.0 - Manual (Page 40)

40 Chapter 5 Working in the Timeline
You can also use the Undo and Redo commands as a quick way of trying out changes
to a song. You can make several changes to the song, then step back through the
changes using Undo. If you change your mind after undoing a step, you can recover
the changes using Redo. You can undo or redo any number of actions, since the last
time you saved. At any point, you can save a new version of the song by choosing
File > Save As.
To undo the last change:
m Choose Edit > Undo.
To redo the last change:
m Choose Edit > Redo.