paying for service I am not getting

I am currently paying for 20mbps adsl line with Telkom but I am getting download speed of 11.38mbps I reported this issue several times technicians said I am too far from the 20 meg port when I call sales department to downgrade I was told there's going to be a fee that i have to pay to downgrade since I am on 24 months contract I do not understand why should i pay to downagrade since its Telkom who failed to provide the agreed service to me, I want to downgrade because I am not getting what i am paying for i want 10 meg line now , I do not see how fair it is to keep paying for the service that I am not getting this is day light ******* by Telkom