Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
That option is not currently available, on the link below are the steps provided to switch to a Microsoft 365 subscription, basically you would be charged for a full new subscription, then the remaining time on the personal subscription would be converted to the Family subscription at a pro-rata basis, in this case 92%
The only other method to switch to the Family subscription would be to cancel your Personal subscription and get a pro--rata refund, then take out a new Microsoft 365 Family subscription.
You can check your eligibility to a refund using this link.