Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
If you have two separate accounts with a Microsoft 365 subscription, if you are cancelling one subscription, ensure that is not the account you currently log into the Office apps with, also, if there are files stored on OneDrive on the account you will be cancelling, ensure you download your files from OneDrive before cancelling the subscription to ensure against data loss, because you will loose access to the 1TB of OneDrive storage on that account.
Then, log into the account with the Microsoft 365 subscription you want to cancel on this link: https://account.microsoft.com/services/
And you can use the steps provided on this link to cancel that Microsoft 365 subscription.