change used photo to used

Hi All

Is there a way to change a used photo back to the unused list? I have deleted a few from my book but they only show up on the used photo list. How can I remove the little green check mark and make them appear on the unused list? I already have the pull down menu set to show me only one list or the other. But I need to move a photo that I previously used so it will show on the unused list.Thanks