how do I replace a single union pressure reducing valve?
A slight hack job, but you should not need a torch, unless the new valve does not have the same thread options.

Wrench big enough to grab the body of the valve itself. To begin with, hold it still.

Wrench to remove the union nut (first) while roundly cursing the installer and inspector who said "Yup, the ground is bonded across this thing" while not providing enough grounding cable to make replacing it easy. You don't want to remove that bonding wire from the pipe, is the thing....

Then a wrench the size of the wrench flats on the fitting above (to hold it still) while using the wrench on the valve itself to unscrew that (there appear to be internal pipe threads as well as the external union threads.)

Reverse the process to install the new valve.