How can I change the order of Autofill in a dotphoto book?

Autofill installs photos in a book from the back of an album. That is: the last photo in the album becomes the first photo in the book.

If you want to autofill your book with photos from the beginning of the album, reverse the order of photos in your album before you go to into the book builder: your first photos will become the last photos in your album, but the first photos in your book.

To reverse the order in an album, choose Organize, then Sort. 

Sort offers many ways to reverse the order of your photos. The simplest is usually Upload Date (oldest first), which is time-sensitive, so the last uploaded photo will become the first in the album, which will make it the last in the book.