Authorise a Mac to play iTunes Store purchases
To play many purchases from the iTunes Store, you must authorise your computer using your Apple ID and password. (Authorisation helps protect the copyrights of the purchased items.)
You can authorise or de-authorise a computer at any time.
Authorise a computer to play iTunes Store purchases
You can authorise up to five computers (Mac, PC or both) at a time. To play a purchased item on a sixth computer, you need to de-authorise another one. An iPhone, iPad, iPod or Apple TV doesn’t count as a computer.
In the Music app on your Mac, choose Account > Authorisations > Authorise This Computer.
If requested, enter your Apple ID password to confirm.
De-authorise a single computer
Make sure you de-authorise a computer before you sell it or give it away. This prevents it from continuing to count as one of your authorised computers and from playing your iTunes Store purchases.
In the Music app on your Mac, choose Account > Authorisations.
Choose De-authorise This Computer.
De-authorise all authorised computers
If you forgot to de-authorise a computer you no longer own, and you have five authorised computers, you can de-authorise all your authorised computers at once. You can use this method once a year.
In the Music app on your Mac, choose Account > View My Account, then sign in (if necessary).
On the right, click De-authorise All.
If you have trouble de-authorizing a computer, see the Apple Support article Authorize or deauthorize your computer for iTunes Store purchases.