Undo and Redo

 Almost any change you make to an image can be undone using the Undo Tool.  Using the Redo Tool then returns the image to the state before the Undo Tool was used. If you want to limit the amount of memory used by the Undo Tool (which can become significant) you set a limit on the number of Undo steps or on the Undo buffer size by selecting Options from the Edit menu.