Apple India — Iphone 7 in warranty

My product is in warranty. Apple offers the best costumer support i have always heard that but unfortunately my experience was opposite. My phone was constantly heating, freezing and vibrating on its own. I went to their store for service they diagnosed software issue and uploaded one. One week after my problems have increased now my phone is rebooting on its own heating freezing. I went to the store again they denies about diagnosis and said we have to send the phone. I am frustrated my phone is in warranty i want refund to my phonr