I'm setting here holding a Sierra projector. The lense is held in a molded FRP (fiberglass reinforced plastic) assembly of two parts. There is the nose portion of the projector that holds the lense out away from the reflector bowl, then there is a FRP part I'll call a retainer that goes in behind the lens and holds the lens in.
In the picture above, you can see the pen indicating where the retainer is held by a melted stud (hot plastic "rivet"). You'll need to cut this off to get the retainer out of the nose of the projector. The nose is also attached to the reflector bowl in the same manner, as can be seen at the center top of the reflector. You'll have to put screws in after cutting off the rivet studs.
None of the work above is difficult. But it seems to me that the projector itself is a poor performing unit, even with HID bulbs. If you go to the trouble to open the housing, either from the front or back, the projector replacement would be a more logical choice.
Many have put the FX-R projector in. But I'm not happy with the fit and the way it has to be modified. I like the RX-350 non-afs BiXenon. The Hella looks like a good fit as well.