More Settings on Your Kindle Fire HD
The My Account option in Settings provides information about the account to which the device is registered. To view your Kindle e-mail address, tap the Learn More About Your Kindle Email Address link. To remove this account from your Kindle Fire HD, from the My Account screen you can tap the Deregister button.
Because the obvious thing to do next is to register your Kindle Fire HD to another account (because so much depends on your having an associated account), you then are presented with a Register button. Tap Register and fill in your Amazon username and password to register the device.