When you do particular things on a device (or on a computer's iTunes) on an account (e.g. using automatic downloads or redownloading past purchases) it will start an association to that account for 90 days : View and remove your associated devices in iTunes - Apple Support
You can either wait until the remaining 31 days have passed, or try contacting iTunes Support and see if they can/will remove the phone from the account that it's currently associated to : https://support.apple.com/en-gb/contact or https://www.apple.com/emea/support/itunes/contact.html
I tunes won't let me download music
