Unhappy customer loosing business

I bought a Tecno W2 phone at a PEP CELL at East Rand Mall now known as East Point on the 06/02/2018 at 18:38. I am currently unhappy with the phone as the GPS on the phone does not work. I am a Taxify driver and the GPS is very essential to me as I need it for work. For instance I add a destination of where I am going and it shows the directions however when driving the GPS does not proceed to the destination but rather stays at the point that i started at so meaning I get lost. I went into the store and advised them of this and was told that there is nothing they can do as the phone is in good working condition. I am still within my 7 days of buying the phone and firmly believe that i am entitled to a refund or exchange, I have no problem with the phone however due to the GPS not functioning as it should and affecting my work please assist as to what I need to do from here