Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
The Microsoft 365 Family version allows you to share your Microsoft 365 subscription with up to 5 family members, the Microsoft Personal subscription does not allow you to share your Office subscription with other family members.
Did Best Buy provide you with a key code card for that subscription, or has that already been added to your Microsoft account online?
If they only provided you with the Microsoft 365 key code card, they should be able to take that back and provide you with the Family subscription version, provided the card has not been used.
If the subscription has already been added to your Microsoft account, then you will need to contact Microsoft Support directly to ask them if it is possible to pay the difference between the Personal and Family subscriptions so your family members do not loose access to Office, try using this method to contact their support.
Go to this link: https://support.microsoft.com/contactus
Select Microsoft 365
Click 'Get Home Support'.
In the box provided, type billing.
Press Enter, then on the next page, scroll down and click 'Sign in to Contact Support'.
Navigate to Microsoft 365 and Office - Manage my Subscription.
Select the 'Chat with support agent in web browser' option