Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hello, good day and thanks for your question. I understand that you are trying to change from the annual to a monthly billing for your Microsoft 365 subscription.
Please take note that you will not be able simply switch as you will have to turn off the auto renewal on the current one and it is either you let it expire and then sign up for the plan again and from there you can choose the monthly payment option.
If it is not letting you do anything or if you are still encountering an issue, you can reach out to the support team at https://support.microsoft.com/en-us/contactus/ and then click on Microsoft 365 and then click Get Home Support and then type in Billing question and click the blue button that says Sign in to contact Support and login and proceed from there.