Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello,
You will need to cancel it via Best Buy because the sale was processed by Best Buy.
See Microsoft's support document here regarding how subscription purchase from third party is managed:
Microsoft explains it here: https://support.microsoft.com/en-us/account-billing/manage-your-microsoft-365-subscription-purchased-through-a-third-party-44749f45-4c0b-5d41-a73f-7634b8d2a763
As for redeeming your 3-month free activation that's included in your new device, you can follow the instruction here: https://support.microsoft.com/en-us/microsoft-365-activation-licensing/office-install/activate-office-for-windows Specifically under this section "Activate an Microsoft 365 purchase or offer that's included on a new Windows device"