Hi
First follow the instructions here:
Get help with the camera on your iPhone, iPad, or iPod touch - Apple Support
If the issue persists, next check whether your camera is eligible for replacement. There is an iSight Camera (rear camera) Replacement Program for iPhone 6 Plus.
Subject to the terms of the program, if the rear camera on your iPhone is producing blurry photos and falls within the eligible serial number range, Apple will replace the iSight camera for you, free of charge. Follow the instructions here: