At the end of a chapter, you will be given an option to either restart the chapter or proceed to the next if available. If the chapter is in progress, you can also restart it by tapping on its cover and selecting the Restart icon (the circular arrow) in the lower right corner of the screen. 

 

A Book can be restarted any time by tapping on its cover and selecting the Restart icon (the circular arrow) in the lower right corner of the screen. Progress will be reset, but any Diamond choices you've made will be saved.