Here you go: iTunes Store: Associating a device or computer to your Apple ID
Removing an associated device or computer from an Apple ID
To remove a device or computer from your Apple ID:
Open iTunes.
Sign in to your Apple ID by choosing Store > Sign In from the iTunes menu.
Choose Store > View My Account from the iTunes menu.*
From the Account Information screen, click Manage Devices.
Click the Remove button next to the device name you would like to unassociate.