Hi you need to DE-register the Kindle first from the existing / previous account and then may RE-register the Kindle to YOUR account. Once the Kindle is setup for your AMAZON account you can access your archive of books from YOUR Kindle (the Kindle if registered will be listed on left under registered devices) or can use the ACTIONS tab online to resend any books to your Kindle.
YOUR Kindle Library contains all of your Kindle content, including books, newspapers, magazines, blogs, personal documents, Audible audiobooks, and active content. You can view titles by content type or use the search box to find specific titles.
Click the Actions button on the right to:
To deregister your Kindle online:
To deregister your Kindle from the device:
To deregister your Kindle Touch from the device:
Once the Kindle is no longer associated with the prior account may then register to yours and have access restored to your ebook library.