by Julieanne Kost
• Command (Mac) / Control (Win) + Z will toggle undo/redo of the last command.
• Option-Command (Mac) / Alt-Control (Win) + Z will step you back through history.
• Command (Mac) / Control (Win) + Shift + Z will step you forward through history.
To change the number of history states (multiple undo’s) that Photoshop keeps track of per open image, select Preferences > Performance and enter a value for History States. Setting a higher number (50 for example) will save more changes, and allow you to step farther back in time, however it will also require Photoshop to keep track of more information. Therefore, if you increase the number of states and notice a performance hit, trying lowering the number again.