Misled,mistreated and rp

A demo phone was sold to me at a full price at Edgar's carnival mall a day ago. I was sceptic to buy a demo but I was convinced by sales assistant by reassuring me that they can exchange the phone within 7 days if it has problems. The packaging of the phone she gave me was obviously already opened And battery was dead. After 8 hours of Charging as per store instructions I got the Worst nightmare of my life with the touch screen keypad not functioning properly. I took it back the very day but the store refused to exchange even when I wanted to pay extra for another new phone instead. The very same sales assistant argued that the phone has A technical fault and at her convenience she used techniques to prove that the phone works. I kept explaining that regardless of those techniques I already had a bad experience and cannot rely on that phone again.I am so disappointed ,I feel ****** misled and mistreated as a customer. I need at least to recover my money.