Sign into your account and open the book you want to edit. In the Contents section, select Front & Back Cover. From there you'll be able to enter your title. There's a 72 character limit. Your title will be front and center on the cover of your book so if you'd like your name to appear there too, be sure to include it.
You'll also be able to edit your book's Side Text, which will appear along the side binding of the front cover. The Side Text has a 54 character limit.