Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
You will need to cancel the Family subscription using the steps provided on this link:
Then, you should be entitled to a Pro-rata refund, you can check your eligibility to a refund using the steps on this link.
Then provided you use the exact same Microsoft account for the new Microsoft 365 Personal subscription, you will have access to all your files and the same 1TB of OneDrive storage.