At any time you can change the number of pages in your book by moving your mouse pointer to the bottom-right corner of the pixpublisher window and click on the +/- icon. This will cause a menu to appear showing the number of pages currently in your book and allowing you to select a different total.
Pages will be added or removed from the end of your book.
If you have already ordered your book you will not be able to edit it. You will need to make a copy of the book and edit the copy.