The Kindle needs to first be de-registered from its existing account. The account owner should go to the "
Manage Your Kindle" page and select the deregister link next to the listing for the Kindle.
Next, log on to the account you want to register the Kindle to, again going to the "Manage Your Kindle" page
for the target account. Under the "Your Kindles" section of the page, select "Register a new Kindle", and enter the 16-character serial number of the Kindle into the field.