How can I replace this water pressure reducing valve?
At 140 psi, either that valve is shot, or you need to install/check your expansion tank. To know for sure, after running some water for a second, stop any running water and check your pressure again. If the high pressure only occurs after the hot water tank has been heating back up, then what you really need an expansion tank.

To replace this valve, after turning off the water, unscrew the union fitting on the bottom of the PRV (at least I'm hoping that's a union, it should be two pieces and the nut should move independently from the pipe). You should be able to work on that with a crescent wrench one partial turn at a time. Then with that disconnected, you should have some play in the pipes to pull it slightly out from the wall and unscrew the valve from the top threaded connection. Installing the new valve will be a similar procedure in reverse, just clean off the threads and apply some pipe dope to the male threads first.