Apple India — Iphone error 9

Address: Chennai, Tamil Nadu, 60000035

I bought iphone 6s in nov 15. Suddenly nov 16th my phone switch off. I restart many times but no use. We go to apple show room they said its software issue so they replaced my phone in dec 16th. But last week also i faced same issue. They said u buy one new phone. What can i do. How many times i faced same issue my husband scolded to me. Please help me as soon as possible

Thanks