Iphone 7 temperature problem

Hello sir / Madam

My mobile phone is iphone 7 and I have a problem with it calls to change it to another and to loss the value of it, the phone temperature of the back side is jump to very high degree within less than 5 minute while using any application even it is video or facebook or a phone call.

So can you please tell me what to do to solve this problem, because i don't want to buy a new one specially with this very bad financial conditions .

regards