Phone that is Faulty

My experience with Apple has been very bad, Last month on the 26 October 2017 I have booked my iPhone 6 plus for repairs coz it was flickering on the screen and then they took it at iStore Cavendish telling me that it will be fixed but i got an email from the technician that were fixing my phone and they told me that the phone didn't have a problem and then today when I went to fetch it, the phone gave me the same problem and they don't want to replace the device that is not even a year old with me.