You can't.
Orion's can have their power reduced to approximately 50% of rated maximum.
As an example, in the photograph is the TRACKING DATA for a 440-470 MHz [100 watt] Orion mobile. The minimum calibrated output power level is fifty (50) watts. Below that, the output power might become unstable, so going below that value is strongly discouraged.
Having said that, you can program any power level between the
50 and
100 (MAX) levels into the radio on a system-by-system basis, by simply typing in the desired power output value in the the . (Portables support channel-by-channel selection.)