- 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 "Deauthorize All".
- Authorize each computer you still have, as you may require.
You may only do this once per year.
After you "deauthorize all" your authorized computers, authorize each one as required.
iTunes Store: About authorization and deauthorization:
One computer using multiple iTunes Store authorizations: