Hello, good day and thanks for your question. 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.
You can also request a call back by going to https://go.microsoft.com/fwlink/?linkid=2168171 in case you are having trouble cancelling and getting a refund.