My first 6s plus had problems after 2 months I got it, were I live there was little to do about it so I just kept using it till the point that the phone started to crash ( color stripes on screen the screen faded and loop logo startup) I noticed the phone did all that when it was hot so I started putting it inside my fridge to cool down and it kept working fine until one day I got it out of the fridge and it wouldn't turn on.
I family member traveled and I sent the phone, she went to a store in Florida and I was able to explain the situation to the employe, I told him everything, they wanted the iCloud account details associated with the phone and ship it for service/repair etc.
They emailed that it was replaced, new phone (white box) at no extra charge.