From the website:

Click on the cover page and select the text layout. From there, you can add or edit the title.

If your book is already in the cart and you need to go back and edit:

Head to your cart and click on the blue pencil icon next to your book to continue to make edits.

From the app: 

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 your book is already in the cart and you need to go back and edit:

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.