You can only deauthorize individual computers from the computer itself. If you no longer have them, then you can only remove them by using Deauthorize All. But that is only usable once every twelve months from when it was last used. So, either you wait until the August anniversary date or remove one of your computers in order to authorize another computer.
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. 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.
For more information on authorization and de-authorization: iTunes Store- Authorize or deauthorize your Mac or PC.