There is a screw hidden under the grip frame that holds the valve in place. Unscrew that, then unscrew the lpc (I think thats what it is) under the barrel, and the whole thing should come out. If not try taking it out the back by removing the hammer, etc.
You will have to wrench it off. Put something soft, but with some grip to it, around it first though. Thin rubber or something similar should do the trick. That will prevent you from scratching it but will allow you to still use pliers, etc.