There's normally a knob on the top with a castellated bottom edge that locks into a mating piece below. Pull it up and turn it. Watch the gauge to see the downstream pressure. There are normally arrows for increasing and decreasing printed on the top. Once you are happy with the pressure setting, push the top down again to lock it in that position.
You'd normally have a gauge before it to show the tank pressure, then the water trap/reg valve with its own gauge, then the hose coupling.