FAQ - Aperture
Can I undo what I just did in the ACM?
These arrow buttons near the bottom-right corner of the screen are Undo and Redo.
Almost any action in the editor can be undone! Up to 30 steps are kept in the undo buffer.
Like many other Windows applications, you may use Ctrl-Z and Ctrl-Y to undo and redo, respectively. Also accepted is the MacOS version of these keyboard shortcuts, Option-Z and Option-Shift-Z.