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 25 undo levels can be set though the General Options section in the File | Options dialog.
Home Tab Commands