Take out the 4 screws around the back end of it. This will allow the barrel nut assembly to come out of the carbon tube
You will see 4 sets of 2 small set screws on the barrel nut assembly. They lock into a couple of channels around the nut. Take out those 8 screws and the barrel nut will come out of the assembly.
Turn the barrel nut onto the upper with the barrel installed. It needs to be to torqued to 30 to 80 lbs, and you will need to line one of the flat surfaces on the nut, the same flats you will use to mount the wrench, so that it is flat on the top. The wide torque range will allow for that.
Slide the housing back onto the nut. Reinstall the 8 small set screws with the holes for the gas tube at the top center. Tighten them down in an alternating pattern, back and forth, as you would when tightening down lug nuts on your car. Install your gas tube and gas nut.
Slide the carbon rail onto it and put back the 4 screws you took out first.