I’ve just got a new iMac and the guy the store recommended I de-authorise media on my old machine. How do I do that

Songs you purchase from the iTunes store are almost all DRM (Digital Rights Management) protected music files. ITunes allows you to play your songs on up to five different computers after authorizing them with your Apple ID and password. If you have recently purchased a new computer or have reached your maximum authorization limit, you can de-authorize songs from one or all of your current computers in order to change locations.

Here’s what you need to do:

Open iTunes.

De-authorize your songs from the computer you are currently on. Select “Store,” and then “Deauthorize Computer.” Enter your Apple ID and password information when prompted.

De-authorize your songs from all computers linked to your iTunes account. Select “iTunes Store” from the left side panel of iTunes. Click on “Account.” Sign in using your Apple ID and password.

Choose “Account” again to adjust your account settings. Re-enter your password and click on “View Account.”

Select “Deauthorize All” under “Account Information” to de-authorize your songs from all computers.

Worth keeping in mind when you change computers, get new devices (like the media player in one of those tv stands), or even a new iPhone.