The only way to edit your photo book is, to edit your footprints and to recreated the book afterwards:
You wish to rearrange your photos. Open the editing dialog of the desired footprint, then tap a photo to move it left or right. The first photo of every footprint will always be the largest photo in the book. The other photos are added in order.
After you are done with your changes, simply press the Recreate preview button of your book (in German “Vorschau aktualisieren”).