I don't believe you can upgrade the camera on any of the phantoms. You can do that on the inspire 1 and 2, but not the phantoms or mavics. If you want a better camera you'll have to upgrade the whole drone.
If you have the original Phantom 4, you'd need to buy a Phantom 4 pro (complete drone) to increase the resolution.
If you have a Phantom 4 pro, you're looking for a much more expensive drone to get any improvement.