There are a couple of ways to change the page order in a book from within the Select Photos and the Arrange Pages screen.
In the Select Photos screen:
- Long press, then drag photos in the tray at the bottom of the screen. The left-to-right order of photos in the tray will be reflected in the book when you tap the Next button.
In the Arrange Pages screen:
- Long press any page and drag it to the desired position in your book. If you drag it to the bottom or top of the screen, the pages will automatically scroll.
Since you can only add one photo per page, you can also use the Edit Page tray to swap photos between pages to reorder. You can learn how to do this here. This is a useful technique when you want to swap pages that are far apart.