How to Deauthorize Your Computer
1) Open iTunes on the device.
2) Choose “Store”
3) Click or Tap “Deauthorize”
4) Type in your Apple Account user name and password to confirm.
There is also a “Deauthorize All” command you can use. This comes in handy if you sold or gave away an old computer or iPad without deauthorizing the device. Just remember when you “Deauthorize All” you must reauthorize all the other devices you plan on using.