I need a refund or my money back

I bought a phone at PEP store at Cavendish Glenmaris I charged the phone 100% Before use. The phone gave me problems with a battery within 2 to 3 weeks. I returned the phone it was booked for inspection for 3 weeks of which I bought the phone so that I can use it for job searching. I got my phone after 3 weeks. I used it only a day the phone started with the battery problem again. I called pep I explained they told me to bring it and it will be booked again for service. So I haven't used this phone not up to a month but I have to be taking it back for inspection until when. I asked if they can change the phone or refund me they told me I can't get exchange or refund am not Happy at all am very disappointed and angry to support a big store that doesn't even care about the clients now it's difficult to use the phone am stuck with it. I really need a phone that I can use since am job hunting..