Undo

Click the Edit | Undo command, click the button, or press CTRL+Z on the keyboard to reverse the last operation performed. If the last operation cannot be reversed, the Undo command is grayed. After you save a file, the undo operations are cleared, so you cannot undo any operations performed before using the Save command. After you have undone an operation, the Redo command becomes highlighted, allowing you to reverse the just completed Undo command.

Undo Levels

Up to 25 undo levels can be set though by clicking the Tools | Project Settings command and setting the Undo levels on the General page. If the Undo command is not available, check the Undo levels. If the value is set to zero, undo will not be available.