Edit
Undo
The Undo command enables you to undo or reverse the previous action or actions. Each time you use a command, its action is added to the top of the undo stack or list. The action at the top of the list is therefore the most recent one you performed and the first one to be reversed.
To undo more than one command in a single operation, right-click the 
tool to display the undo stack.
Click on one of the items in the undo stack to undo that action and all of the actions that come before it (above it) in the list.
If there are no actions that can be undone, the undo tool is grayed out (disabled).
You can control the number of actions that can be undone (the size of the undo stack), from the General category of the
System Options.
| Note Some commands cannot be undone. When you select a command that cannot be undone, a message is displayed to tell you that the action you are about to take cannot be reversed. The command causes the undo stack to be completely emptied. |