You can edit the title of your 5x7 Monthbook in the Chatbooks' app. (Note: The 5x5 Monthly Minis' title are not editable.) To edit the 5x7 Monthbook title, you do need at least 1 photo in the book to adjust the title. Then, follow these steps:
Login to the Chatbooks' app, and go to the first tab (for iOS Users, it is under the My Books' tab).
Tap into your Subscription.
Click into the Monthbook you want to change the title on
Under the cover, tap the Pencil icon to edit your title
Enter the title you want
Click Done (on an Android device, instead of "Done" you will see a checkmark)
Your new title will then appear on your Monthbook.
Here's a video of how to edit the title of your Monthbook as well: