Apple India — Iphone 5s handset service issue

Address: Ranchi, Jharkhand

I bought i phone 5s on june'16 and in june'17, phone get the issue of being switched off suddenly and gets heated badly, i went to f1 info solutions & services, ranchi on 20/06/2017 and submited my phone for repairing under warranty period,
On 10/07/2017, i offered a brand new phone against my old phone, which was issued of hardware and service boy replied that, this is not been treated so, we are offering new phone to you,

But after getting new phone, again the new phone has same issue, it gets heated badly and switched off suddenly, i didnt understand how come the same issue with brand new phone,

Kindly let me know, what next i will do.

Solotion?