I want to change my monthly payment

You'll need to contact Adobe Support to cancel your Annual (paid monthly) plan and resubscribe to an Annual (prepaid) plan.  There is a slight cost savings, too. 🙂

If early termination fees apply, the Adobe agent can probably waive those for you.

Hope that helps.

when you contact adobe support per above be prepared to cancel your current plan and restart a new one thereby restarting your one year term.

If I understand you well, you want to change from monthly payment to Yearly payment? And you have an individual licence? Like Creative cloud all apps? Contact support. Adobe customer care can be contacted by beginning a secure chat session at https://helpx.adobe.com/contact.html?rghtup=autoOpen. Pop-up blockers need to be disabled! If the chat window fails to open, or is non-responsive, use a different device to start the interaction.
Support phone numbers may be found here: https://helpx.adobe.com/contact/phone.html. Please note that phone charges may apply.

For support via Twitter:
https://twitter.com/AdobeCare

(see also here: https://community.adobe.com/t5/creative-cloud-services/how-to-contact-adobe-support/td-p/11875703 or here
https://community.adobe.com/t5/account-payment-plan/how-to-contact-adobe-support/td-p/11843852)