You need to first deauthorize all computers, then re-authorize the ones you currently use.
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.
After you "de-authorize all" your authorized computers, you may 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.
For more information on authorization and de-authorization: iTunes Store- Authorize or deauthorize your Mac or PC.