How to Deauthorize an iPhone on iTunesby Matt Milano
When upgrading iPhones, users often trade in or sell their existing model to help offset the cost of the new device. You should always deauthorize your old phone in iTunes before getting rid of it to ensure no one else can access your purchases, personal data or iTunes account. You should also deauthorize your iPhone if it is lost or stolen.
Open iTunes on your Mac or Windows PC.
Click the "Store" menu and select "View Account."
Enter your Apple ID and password and then press "Enter."
Scroll down to the section labeled "iTunes in the Cloud."
Click "Manage Devices."
Click "Remove" next to the device you want to deauthorize.