I want to upgrade from individual Apple Music to family one
Upgrading family sharing Apple Music from individual plan
See the following from https://support.apple.com/en-us/HT205595
If you already subscribe to Apple Music, you can change to a Family Membership in the Music app or in iTunes on a computer. Learn how to manage your Apple Music subscription on your iPhone, iPad, iPod touch, Mac, or PC, or on an Android phone.