To change or edit the title of your book, view the book cover, then tap the page to enter the editing mode and double tap on the title to edit. (If it's hard to tap, just tap the title or grab a stylus for accurate tapping.)
If your book is already in the cart and you need to go back and edit:
From the app: Just head to your cart, tap the book, tap the cover page, and select one of the two text layout options. Once you've done that, tap where you see the pencil icon to add a title.
From the web: So sorry, once books are in your cart, you can't edit them. If you need to make changes, you'll need to start fresh.