If you’re already subscribed to Apple Music, you’ll need to cancel directly with Apple in order to enjoy membership.
https://support.apple.com/de-ch/HT202039
After you have cancelled your current Apple Music subscription, you need to text MUSIC to 5522 or log in to My Sunrise, select your subscription go to the Options section to add Apple Music.
And don’t worry, you won’t lose your playlists and favourites, as you will sign back in with your existing Apple ID