Hi Talha,
@ds17 is correct; the cameras for each model of iPhone are custom built for that phone and that phone alone and will not work in another phone. There are differences in connector size and position as well as the camera mounting, and its unlikely that the phone's software would correctly recognize a different camera even if you were able to physically replace it. And we haven't even begun to address the new image stabilization hardware that comes with newer cameras.
So the short story is, no, iPhone cameras are not upgradeable. The closest you can get is to upgrade your phone to a newer model.