Authorization and Deauthorization
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.
More On De-authorizing Computers (contributed by user John Galt)
You can de-authorize individual computers, but only by using those computers. The only other option is to "de-authorize all" from your iTunes account.
1. Open iTunes on a computer
2. From the Store menu, select "View my Account..."
3. Sign in with your Apple ID and password.
4. Under "Computer Authorizations" select "De-authorize All".
5. Authorize each computer you still have, as you may require.
You may only do this once per year.
After you "de-authorize all" your authorized computers, re-authorize each one as required.
If you have de-authorized all computers and need to do it again, but your year has not elapsed, then contact: Apple - Support - iTunes - Contact Us.
Note: Authorization and Deauthorization only applies to Computers.
If you no longer have, or access to, the computer(s) you want to deauthorise,
Log in to iTunes, go to "view your account info" on the itunes store, deauthorise all five,
(Please Note: this can only be done Once every 12 months)
Then re-authorize your current Computer(s) one at a time..
Authorise / Deauthorise About