Follow these instructions from Apples Knowledge Base:
Click iTunes Store on the left side of iTunes.
If you're not signed in to the store, click the Account button, then
enter your account name and password.
Click the Account button again (your Apple ID appears on the
button), enter your password, and then click View Account.
In the Account Information window, click Deauthorize All. *
Note: You may only use this feature once per year. The Deauthorize All
button will not appear if you have fewer than two authorized
computers. If you need assistance on using this feature, please
contact iTunes Store support via email