Miscellaneous topics that do not fit into specific categories.
Log into any Microsoft account you may have on this link, do you find the Microsoft 365 Family subscription listed there?
https://account.microsoft.com/services/
If you find the subscription listed there, select it and click Manage, you should then be able to cancel the subscription and disable the recurring billing on the subscription to avoid any further payments on the subscription.
When you cancel the subscription, you should then be entitled to a pro-rata refund for the last subscription payment, you can check your eligibility to a refund using the steps on this link.