Photoshop’s History Panel
Working with the History panel is a bit like Try-before-you-buy. You try something out, and if you don’t like the result, you simply give it back. Photoshop has very sophisticated mechanisms for undoing things you have just done.
Photoshop notes the last n steps you have made, according to settings you make in the general Preferences. We usually select a value of 25. You can then undo (and, if you don’t change any settings, redo) each of the last n steps sequentially. You can use the History panel not only to reset your work to an earlier state, but also to compare earlier versions of your image with the current state.
In order to return to a particular state, simply click on the appropriate state in the History panel or ...