In Photoshop, undoing works a bit differently from most other applications. You can press Ctrl+Z (or Command+Z on a Mac) to undo your most recent change. However, if you press Ctrl+Z again, the change will be redone. If you want to undo several changes in a row, you'll need to select EditStep Backward or press Ctrl+Alt+Z (Command+Option+Z on a Mac).
After you've made a few changes to the example file, try using the Step Backward and Step Forward commands to see the effect.
After you've edited an image, you'll want to save a new version of the edited file in order to preserve the changes. This will also prevent you from accidentally overwriting your original image file. There are many different ways to save files in Photoshop, which we'll discuss in detail in the next lesson.