To remove the rear light reflector, simply undo the two screws securing it in position. If it has any sign of damage, discard it. Replacements are readily available.
The housing itself, is secured with two threaded studs that are fixed to the housing and then inserted through the frame canopy through two locating holes. Below, you can see the nut in position, holding the housing in place.
IMPORTANT. Do not at any point disturb or remove any of the wiring unless you absolutely have to. This can be removed much later and a series of pictures taken to illustrate exactly how it is all routed correctly. There is no need to remove any of it at this point.