With the light not turning on, that leads me to ask if the marker will fire at all. Usually if that led is dead, your board is shot as well. You may end up having to get a new board.
With that being said, to change the co2/hpa settings on a stock board is very simple. Disconnect the battery and hold down the power button while reconnecting the battery. If it blinks once, it is set for co2. Reconnect the battery again with the power button held down to make it blink twice for HPA.
Sic gorgiamus allos subjectatos nunc.