Changing Tarrif of 24-month contract after 12 mon

During April 2014 I was telephonically contacted by Vodacom Marketing and promised that if I change my \Talk 500"contract (which I had for many years) to a new 24 month XL-bundle contract I would save a lot of money on the new contract because it includes a data-bundle. NEVER EVER!!! did the marketer told me the contract-fee will increase after 12 months. I have received a SMS from Vodacom informing me that my contract-fee will increase from 01 May 2015. My questions are : 1) How can I get out of this Contract"