Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
What subscription do you have now? When you said downgrade, I assume you must be on the Premium plan then?
If so, turn off auto renewal for the current plan. Please go to https://account.live.com
Sign into your Microsoft account. Then go to Services. Click on Manage for the subscription plan to do so.
Let it expire. Then on the day it expires, you sign up for the Family plan separately. Make sure you use the same Microsoft account so you don't lose data.