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 deauthorise a computer anytime.
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.
Deauthorise a single computer
Be sure to 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 > Deauthorise This Computer.
If requested, enter your Apple ID password.
Deauthorise all authorised computers
If you forgot to deauthorise a computer you no longer own and you have five authorised computers, you can deauthorise all your authorised computers at once. You can use this method once a year.
In the Music app on your Mac, choose Account > Account Settings, then sign in (if necessary).
On the right, click Deauthorise All.
If you have trouble deauthorising a computer, see the Apple Support article Authorise or deauthorise your computer for iTunes Store purchases.