LG Mobiles - No resolution for my Nexus 5X

I am a call student I have purchased Nexus *x for rs ***** , My phone was running smoothly for * years after that it stopped working due to a software update from the lg the phone started switching on and off repeatedly. I took the phone to the service center they told me after update software one it didn't worked well after working well for one hour again it started now when I complained regarding this they told me to pay ***** for motherboard change when I did nothing to my phone what's my fault there is a fault in this phone already that's why this issue is arising I don't know what to do lg I need refund as I got a defective phone and no one is helping me I am a student I don't have enough to spend on your faulty phone. I need total refund for my phone please refund or exchange this faulty phone.I wrote a mail to expect a reply I need refund for it.