Sorry, my apologies, I forgot to tell you that you need to cancel your current subscription and then upgrade via the family plan 🙂
to unsubscribe from premium follow these steps:
Canceling your subscription reverts your account to the Free level.
Your subscription page now shows the date you will revert to our Free ad-based service. We hope you decide to upgrade again!
Then you can sign up for Spotify Family here.
I Hope this helped! 🙂