I have dust on my camera too and is annoying af, sometimes i take pictures of stuff i want to sell and people see them as cracks or defects... If its a replacement, you could turn that one in too, iphones 6 are really hard to open...
if its still under warranty, you can send it back. but the quickest way is to go to a phone shop and it takes less than 5 mins to open that section and clean it. cost very little amount for the service.