Thank you for writing to the Mixbook team! :)
We have a tutorial on how you can move pages within a book. This tutorial covers how to swap pages and also how to move a two-page spread to another location. Please visit our tutorial here:
Please note that at this time, if you delete a page in your book we do not automatically move up all of the pages to fill in the blank page. You will either need to delete the second associated page in the spread, or swap an alternate page in your book with the blank page. (Any pages left blank in your book will print out that way.)
We hope this helps! Please feel free to contact us again with any further questions or concerns!