Apple Warranty

i bought an iphone 6s plus from 25th December 2018 and just 2 months later the iphone's screen had a problem: the top right screen was darker than the rest so i set up a repair and have just received the phone last wed ( 20/3/2019). However Apple didn't repair it thoroughly and the issue come back AGAIN which is really annoying hence all of my work is on the phone and Apple took so long to repair last time ( almost a month). I really hope they could offer a better solution for me