Remove the 6 Phillips screws on the front of the lens mounting ring.
Carefully lift the mounting ring straight up.
Three unfastened metal plates lie under the lens mounting ring, be very careful lifting it up as to not disturb them.
Reseat or replace the lens mounting ring and reassemble.
The locating pin on the right of the camera will help place the mounting ring properly.
The small metal plates marked in the image are not fastened and may displace if the camera if tipped or bumped. Be sure these are properly in place before reassembly.