It will be very near the main shut-off valve for the house. Locate that valve and 'follow' the pipe with your eyes till you reach the water softener or water heater. There are several models in common use, but the best, in my opinion, is the Watts 25AUB. It has two connections to attach to male adapters on the pipe itself, and a large brass cone shape perpendicular to the water line. On the small end of the cone there is an adjustment and a lock nut to adjust the pressure if you don't like the standard 50# p.s.i. set at the factory. Pressure-reducing valves fail in two ways:
They eventually will 'break' inside and allow full line pressure to pass through. This requires replacement of the unit. Test your water pressure past the pressure-reducing valve. If it exceeds 50# p.s.i. try to adjust the pressure on the valve. Be sure to leave one faucet open enough to get a current reading of the pressure after the PR valve.
They can eventually plug with debris. This will cause your water pressure to be much lower when a second or third tap is opened. If your pressure drops more than 10# p.s.i. when you open a second faucet I suggest you replace the pressure-reducing valve. Replacing with an identical unit is much easier, so try to buy exactly what you have.