Don't know how to replace a screen properly

Two weeks ago I took My galaxy S5 on a monday to have the screen replaced as My screen had cracked,the phone was under samsung,s two screen repairs warranty,we were told the they would call Us as soon the phone was ready,by the Friday I had heard nothing so I called them and the phone was ready so I drove all the way to Rivonia to pick up the phone,took it home switched it on and noticed the screen was pale green instead of white and all the colours were dal and after a while the screen would go completely black,so I took it back on the Monday and again was told by the technician I had to re book it in again,so I sat and waited in long que to rebook it in again only to be told the same story that they will call Me once it's ready and once again got to the Friday and still hadn't heard from anyone so this morning Sat 26 Sep 15 called them and was told the phone was ready,drove all the way back and collected the phone tested it there and it was still doing the same thing so I was told they had to loosen the screen,drove home and tried to set the phone up and of course it's till doing the same thing again,obviously they don't know what they are doing,this is pathetic I mean really.