If you have two to four similar photos, you can compare them side by side to see which one you like best. Or, while you’re editing a photo, you can compare the edited version with the original to see if you want to keep your changes.
Select two to four photos you want to view or edit.
Click the Edit button in the toolbar.
After you make changes to a photo (and are still in edit view), press the Shift key to see how the photo looked before you changed it. When you release the Shift key, the edited photo reappears.