How to change service plan?

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 merely want to downgrade my account.  PLEASE RETURN MY CALL!!   1-702-539-6839

Hello Xilium, 

You will want to speak with your Account Manager or you can contact our Sales department :)