Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Is the current one purchased from Microsoft directly, or was it from Best Buy too?
If it was from Microsoft, Please go to https://account.live.com
Sign into your Microsoft account. Then go to Services. Click on Manage for the subscription plan and turn auto renewal off. This way it will expire on August 12. Then on that day, you can redeem and activate the Best Buy one.
And yes, I do recommend that you wait til August 12 to activate the family plan so you don't waste the overlapping days.