It's internally threaded and should come off with relative ease. It the threads were likely galled on install. I'd try a liberal spray of power lube, WD40 or some similar penetrating fluid. Soak it down real well and then let it site for an hour before trying to unscrew it.
If that doesn't work (if it's galled bad it won't) you'll have to take a torch to the PRV or use a cut of wheel to take it apart. Try not to touch the threads if cutting it off. Just come close and then use a cold chisel to open it up. Get a die, can't recall what size the Mytton/Rodd takes, and chase the the threads before installing a new PRV and use an anti-seize on the threads.
If you're just lookng to clean it and the PRV is holding pressure you might just want to let it be. Of course that just means it will fail to hold pressure when you're really counting on it and will have to deal with it then.