The options to change the cover, size, paper type and thickness are only available for the Layflat Photo Book, Slim Photo Book, Flush Mount Album, and Fine Art Album.
To change the cover type, size of the book, paper type or thickness, click on "Options" at the top left side of the screen next to "Photos."
If the option you would like to change is not listed, then that option can not be edited. You will need to select another book type. The edits you have made to the current book will not transfer to the new project.