By default, all your photos in the book are in chronological order, based on the date the photo was created or uploaded. There are three ways you can change this order:
In Preview, you can drag and drop any photo over another photo on the same or a different page and they will be swapped;
Or simply hover over a photo on a page in Preview and it will highlight all the options available to change the page, including replacing the photo.
Alternatively, click the Edit button next to the Order button at the bottom of the preview to access the edit page. Here, you can select the photos you want to manage: hide, return, rearrange, and upload more photos. Once you have made your changes, click Apply to rebuild the album.