Undo:
From the toolbar, select the left-pointing arrow 
to undo your last action. Press multiple times if needed to undo multiple actions.
Redo:
From the toolbar, select the right-pointing arrow 
to redo your last action. Press multiple times if needed to redo multiple actions.
Tip: You can also use a keyboard shortcut to undo an action - Undo:
Mac: Command + Z
Windows: Ctrl + Z