The Beretta 92FS has a nifty CO2 venting feature for when you've finished shooting, which means you don't risk frostbitten fingers when removing the CO2 bulb - you can be sure that the bulb is completely empty before attempting to change or remove it.
a) Insert EMPTY rotary mag and close the breech.
b) Cock the gun by pulling the hammer all the way back with your thumb.
c) Pull back some more on the hammer until it won't go any further back, and whilst keeping hold of it, release the trigger and allow the hammer to return GENTLY.
d) Keep the pressure on the trigger, and you'll notice that the hammer has gone back, but still stands a few mil proud of it's usual position.
e) Still keeping the pressure up on the trigger, use one or both thumbs to press the hammer all the way into it's closed position. You should hear the gas venting harmlessly, and you can now safely remove the spent CO2 bulb.
Hope that helps, and happy shooting.
Mike.