Remove the grip frame, there is a brass screw holding the valve in place. You also have to remove the recock screw first. Once those are out, it should move pretty easily with a push from the eracer end of a pencil.
Newer OMENs the valve retaining screw might be aluminium not brass.
When you put the valve body back in you have to line it up first and slide it in carefull so as not to spin it. When the hole lines up screw in the retaining screw. really not hard once you've done it.