Authorization
iTunes Store - Authorize or deauthorize your Mac or PC.
In iTunes you use the Authorize This Computer or De-authorize This Computer option under the Store menu in iTunes' menubar. For Windows use the ALT-S keys to access it. Or turn on Windows 7 and 8 iTunes menus: iTunes- Turning on iTunes menus in Windows 8 and 7.
Deauthorization
You can de-authorize individual computers, but only by using those computers. Your other option is to "de-authorize all" from your iTunes account.
- Open iTunes on a computer
- From the Store menu, select "View my Account..."
- Sign in with your Apple ID and password.
- Under "Computer Authorizations" select "De-authorize All".
- Authorize each computer you still have, as you may require.
You may only do this once per year.