Undo
Click the Home | Undo |Undo command, click the 
button on the Quick
Access Toolbar, or press CTRL+Z on the keyboard to reverse the last
operation performed. If the last operation cannot be reversed, the Undo command is disabled.
After you have undone an operation, the Home | Undo | Redo command becomes highlighted, allowing you to reverse the just completed Undo command. Up to 200 undo levels can be set in the General section in the Options dialog.