Most of the Adobe creative applications feature unlimited undos. And to access these multiple undos, you can use the command-z (Mac) and ctrl-z (Windows) shortcuts. To redo, use command-shift-z (Mac) and ctrl-shift-z (Windows). However, Photoshop is different. Photoshop offers only a single undo. But, in its place, it features a History Panel, allowing you to move forward and back in history. But you can change Photoshop's keyboard shortcuts so that undo works like you'd expect it to. Here's how:
1. Choose Edit > Keyboard Shortcuts
2. In the Keyboard Shortcuts dialog box, click on the triangle to the left of the word Edit to spin the Edit category open.
3. Click in the Shortcut field to the right of Undo/Redo, and press the delete key to remove the keyboard shortcut assigned to Undo/Redo.
4. Click in the Shortcut field to the right of Step Backward, and press command-z (Mac) or ctrl-z (Windows)
5. Click the Accept button.
6. Click on the Save icon at the top of the dialog box, give your shortcut set a name and click the Save button.
Now, command-z (Mac) or ctrl-z (Windows) will step backward through the History panel, giving you multiple Undo capability. Add the shift key to step forward through the History panel.