Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello, good day and thanks for your question. Sorry to know about this and I understand that you are looking into cancelling your subscription as you forgot to cancel it.
As you have an existing subscription, you will need to cancel it first and then you can go to https://account.microsoft.com/services/microsoft365/billing and try it from there or try to turn off the recurring billing at https://go.microsoft.com/fwlink/?linkid=2277073 and you can also follow the guided steps indicated from the link below which also includes refund eligibility check.