Use Undo
to reverse the last action. If you are not satisfied with the result, click Redo.
Each time you use Undo the undone action becomes a Redo. For instance, you might erase a cross and then Undo the erase. You could now Redo the erase or do something else. If you erase a different cross instead of using Redo then the Redo is lost, along with any other Redos that may have followed.
There are unlimited Undo steps. Undo steps are only lost when you close a design.
Shortcuts
Quick Access Toolbar:
Keys: Ctrl + Z; Alt, 6