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.
Note: Removing a device from your Apple ID does not override the 90 day timer. The timer must complete 90 days from the day the device was associated before it can be associated to another Apple ID.
Information taken from Apple Support Article HT4627