To play many purchases from the iTunes Store, you must authorize your computer using your Apple ID and password. (Authorization helps protect the copyrights of the purchased items.)
You can authorize up to five computers (Mac, Windows, or both) at a time. To play a purchased item on a sixth computer, you need to deauthorize another one. An iPod, iPhone, iPad, or Apple TV doesn’t count as a computer.
You can also authorize up to three computers to play items purchased from the Audible website (audible.com). Audible is a third-party company that sells audiobooks and other spoken programs.
You can authorize or deauthorize a computer at any time. Be sure to deauthorize a computer before you sell it or give it away.
Authorize a computer to play iTunes Store purchases: Choose Account > Authorizations > Authorize This Computer.
Deauthorize a computer: Choose Account > Authorizations > Deauthorize This Computer.
Deauthorize a computer to play spoken programs purchased from the Audible website: Choose Account > Authorizations > Deauthorize Audible Account.
If you forgot to deauthorize a computer you no longer own, and you have five authorized computers: You can deauthorize all your authorized computers at once. You can use this method once a year.
Sign in to the iTunes Store, choose Account > View My Account, then click Deauthorize All. If you don’t see this option, it’s because you have fewer than five authorized computers.
If you have trouble deauthorizing a computer, see the Apple Support article Authorize your computer in iTunes.