iPhone 6 Plus rear camera not working

Where are you located? Are there no Apple Stores near you? Because iPhone 6 is still under warranty at this date.

So if no one opened and tempered with your iPhone,

and/or you did not drop it and it looks (seriously) damaged,

and/or you did not spill any liquid on it,

Then you can have the camera fixed or the iPhone replaced for free (whichever applies in your case) under warranty and the parts used are genuine. It pays to try either way.

If you insist on bypassing the warranty, you can follow this guide (it will tell you the needed parts, tools, and steps) to replace the rear camera. But your warranty will be void.