Bad service

I am not happy with telkom & Samsung I bought a phone and within 7 working days it gave me problems took it back to the store and was told no they can't give me a new phone saying I returned it on the 8 day so they can't give me a new one as they were referring to their seven working days because they work Monday to Sunday. So I was fine they took the phone and after a week it was returned and was told that it was software problems but then I could see nothing was done as the same problem persist I told them and the guy at the shop requested to keep it and check what might be the problem after a week he said I must come fetch my phone but when I get home the problem problem was still there. So now it 2 months and I am paying for a phone that I am not using and one is giving me a feedback. Please help how can I cancel this contract