Apart from the options in the ”Customize” section, the only way to edit your photo book is, to edit the included footprints and to recreate the book preview afterwards:
You wish to rearrange your photos. Open the desired footprint, change the order of photos via drag and drop and save it. 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 underneath the preview of your book.