In addition to what Saadet just mentioned, if you are speaking about changing from RingCentral Office Standard edition to the Premium or Ultimate edition, you can do it in the Billing section of the Admin Portal by clicking on the
Change Edition button (see the screen shot below). The reason I mentioned this option is because sometimes customers can confuse an "edition" with a "plan".
If however you want to change from the Professional plan to the Office plan, you would have to contact your Account Manager.
I would like to downgrade my service and after contacting sales they said only my account manager could perform action. Still have not heard from my account manager. Is there a reason RC makes this so difficult?