Every Kindle has a specific Amazon account associated with it, which is the email and password that the owner uses to log into Amazon. You can change this account to a different Amazon account at any time, if you registered with an account that you do not use. Changing the account will not result in losing any of the content on the Kindle.
Click the "Menu" button while on the home screen on your Kindle.
Click "Deregister." Confirm that you want to deregister the Kindle, by clicking "Deregister" in the pop-up box.
Click "Register." Enter the email and password of the new Amazon account.
You can also change your Kindle's account from Amazon's "Manage Your Kindle" webpage.