Foneexpress

I bought a phone from a fone express outlet at west gate. The phone got spoiled in three weeks. Fone express a.k.a CON EXPRESS told me that they would replace or upgrade my phone for me if the phone ever had a problem during the warranty period. I went to get an upgrade and they refused to change my phone. They said they would repair the phone instead. I went back the next day to collect the phone and the technician had made the phone even worse. The phone would not even switch on. He had the nerve to lie to me and tell the phone was fixed and that I must sign a form saying that the phone had been fixed. Luckily I checked the phone. Now they say they will give me a new phone. When I asked them to change the phone with a different phone they refused even though I was prepared to pay them extra for a new phone.
   They started giving me rubbish exscuses like I used the phone for more than 48 hrs and it has a tiny scratch so they can't change the phone. Till now I dont have a working phone. 
CON EXPRESS are bastards. Any ideas on what action I can take against them?
They are shameless liars. Hope they burn in hell.