Iphone 7 plus screen keeps going black and touchscreen lagging?

I'm less than a month into my new contract for the iphone 7 plus and the phone is already being faulty, the screen goes black more so when the phone is tilted to the side or when on some apps or games and the touchscreen also has become faulty, when locked i can't use the touchscreen at all i have to unlock it, and it also lags sometimes, i've been told to take it into my local O2 store, what exactly will they do replace it or fix it?

So return it.