Okay... technically you aren't supposed to touch the relief valve, but since you've got two you could try this on your "non-competition" one and then figure things out from there.
This is, after all, a safety issue and what you are doing will make your robot more safe.
The valve's release pressure can be adjusted by turning the "head" of the valve relative to the "body" of the valve after releasing the "lock nut" that keeps the head from vibrating loose.
Theoretically the valves come pre-set to 120psi, but at least once we have had a similar situation and corrected it to ensure that our tanks did not exceed 120 psi.
Congratulations on identifying a potential safety hazard and pro-actively taking steps to remedy the situation.
Jason
P.S. Also remember to set up the pressure switch and wire it in to your RC so that the compressor only turns on when needed. There are other threads on CD where you can find help with the (really, really simple) code needed to do that if you haven't programmed it in already.