If you ever make a mistake in Photoshop, there’s a good chance you’ll want to undo it. Here’s how to do it in Mac OS X:

  1. Open the Photoshop application.
  2. Select the object you want to undo (or select all objects if you want to undo all changes).
  3. Click the “Undo” button (it looks like a red arrow with a white “U” inside it).
  4. The “Undo History” panel will appear at the bottom of the window, listing all the changes that have been made to the selected object since Photoshop was last opened. To restore an earlier version of the object, click on one of the thumbnails in this panel and choose “Restore Previous Version…” from the menu that appears. ..