How do you depressure this? The instructions say something about a Rigger and the illustration is unclear to me.

You turn the tank upside down and squeeze the spray actuator. This sprays the liquid out of the line and then releases the tank pressure. Much safer than just unscrewing the pump handle to release the pressure.

Where is the “pressure release valve” located ?

Bill, i am not sure what model number sprayer you have. If you have a sprayer with a pressure release valve, you would turn it to the right one click to release the pressure. If the sprayer does not have that option you will slowly turn the pump away from you counter clockwise and that will release the pressure.