Either a Logic board issue or battery. It could also be a faulty cam depending where you notice it getting hot . If its the back by the logic board probably something wrong with it. Could be a bad Battery or cam as well, or all the Above. Hard to Diagnose without it in front of me, but these are the parts I would try first , if replacing doesn't help then most likely the logic board, time to upgrade to a newer phone if that is the case. Hope this helps. Also, I agree with Minho apple will not attempt to work on this phone since its an older model now they will offer a trade in or something most likely, if not an out of warranty replacement. But I would start by seeing which part is overheating and then start replacing the parts .