Removing the display assembly is the first step to most iPhone 6 Plus tinkerings
Rear camera on iPhones doesn't seem to have any gaps for dust to get in, but for some reason it gets in on rare occasions. You could take the phone to an authorized service center for either cleaning or refurbishment, or if that's not a viable option like my case, you could do the cleaning yourself.
To do that, you need to first open up the phone and separate the display assembly from the rest of the phone. The cables connecting the two are held by five screws and a cover plate, so once you take them out, the four cables can be popped off with a spudger.
Remove the cover plate to pull out the rear camera
To access the rear (back) camera, you need to remove two screws that hold the cover plate for the camera on the top right area of the phone. The screw on the left is obscured by a black pad, so you need to lift it slightly with something like a flathead screwdriver while you unscrew. The cover plate may not completely become loose, but it does not need to be.