Paid for a 6Mb and got 2Mb

I sent an email at the beginning of the year explaining them that I needed a 6mb line, they rep**** to me, we did the contract over the phone bla bla, I needed the internet to work so as I was using it and noticed it was extremely slow, contacted them and apparently I only had a 2Mb line, despite the fact that I, in written, asked for a 6Mb. So in May I got \upgraded"to the 6Mb line"