Looking at the way it is plummed, I would say you would open the valve next to the reg, while the other valve(vent) is closed, to send regulated air pressure to the tires to inflate.
To deflate you could simply lower the reg pressure and the tire air would bleed back thru the regulator vent(slowly) till the new lower pressure is reached. the problem here is that since the gauge on the reg is reading the output pressure of the reg, you dont actually know what the reg is set at untill the tire pressure bleeds off. so you would probably want to close the valve next to the reg, set the desired lower pressure on the gauge, then open the valve and let the tires bleed down to the now known setpoint.
Or you could close the valve next to the reg, open the other valve to vent the system then close that valve. Set 8-9 PSI on the reg and open that valve which should be enough to open the wheel valves and dump valves which would deflate the tires rapidly untill you open the vent valve and close the reg valve to close the dump valves under teh truck and halt the process. Then set a higher pressure to open the valves and check the tire pressure... that sounds pretty cumbersome, way easier to deflate the other way IMO, probably just not as fast though...
