If you want direct access to Microsoft Support just do a web search. You'll find it. Your question, though, sounds like a Mac App Store subscription question. I would expect to see that in the App Store under your account (bottom of the left column), then click View Information (top right), and finally find the subscription management options at the bottom.
This doesn't relate specifically to your question but I just switched an existing Office installation from the Microsoft downloaded install to an App Store install. It's the same Office 365 account. Here is what I did:
1. Opened Microsoft Word and signed out from my Office account (third item down under the Word menu).
2. I followed Microsoft's instructions for uninstalling office:
Uninstall Office for Mac - Office Support
This involved:
a. Deleting the office apps from the Application folder
b. Deleting all Microsoft related files from the system Library folder and user Library folder
3. Reinstalled the apps from the App Store.
4. Upon launching each app for the first time, I...
a. Granted access to the Mac keychain (Always allow)
b. Signed in with my Office 365 credentials
c. Denied access to my Contacts