This feature is only for the Layflat Photo Book, Flush Mount Album, and Fine Art Album.
Click on "Options" at the left side of the screen next to "Photos". On the "Options" tab, you can change your options available to your photobook such as book type, cover, size, paper, etc.
If the option you want to change is not listed, then the option is not changeable. You will need to start over and select another book type.