When the book preview is created, click 
in top right corner. Here you click on the 
in top right corner and you’ll land on the editing page. If there’s a photo you want to replace, you click on it and select 
at the bottom of the page. Now you can scroll through the camera roll and when you find a photo you like you click on it and to save, click the check mark in top right corner. Done!
If you have any questions, please don't hesitate to contact us!