Tmoblie Tell charge me for service I didnt have for two years they been over charging me and they wont give me my money back Atlanta Georgia

I cancel a extra service that i had on my phone over the phone to a customer service rep. She told me that the service would be remove. I thought that the service was remove that was two years ago. I found out that the service was never remove i been pay for that extra service i no longer want. that extra service cost $20 dollars extra a month having paid that for two years now that $480 dollars total I pay to tmoblie. I want to know how do i get my money back.

Stlyesbyeve
riverdale, Georgia
U.S.A.