It's either clamp-style or
Heyco, I'd imagine. If clamp-style, it should be fairly obvious, with two screws you loosen to lessen its
grip. The entire piece has a locknut on the back that unscrews for removal.
If
Heyco, I second
@derekleffew 's suggestion in getting a set of the pliers. I have a set and it is one of my least-used yet most valued tools. One, because it was a
bit pricey, but also because it makes that job SO much easier.
In the off chance that it's a cable gland (google it), the entire thing has to unscrew from the
fixture's housing first. There should be a
nut on the back and a place to get a wrench on the front. When it gets removed the inner jaws will open up. This style of
strain relief is common on the cheaper aluminum DJ-grade cans.
Edit: I'm going to correct myself on the cable gland explanation. Been a while since I installed one, but I think the "
dome" portion is separate from the assembly that feeds through the
fixture housing. Clamp your
c-wrench on the hex at the bottom of the gland (against the outside of the housing) and try a pair of
channel locks to unscrew the two halves.