...with a hammer (gently at first of course, then gradually more forcefully) should release it. You can do this best and most conveniently while the valve is still in the gun.
It's also possible to release pressure by tapping the checkvalve with a pin punch inserted though the intake hole, if the firing valve has failed in a way that prevents it from opening even when the stem is depressed. That's possible and sometimes happens if the rubber seat slips out of the stem head.
If all else fails, you can also (carefully!) unscrew the valve halves. The seal "should" break and the air escape before anything violent happens.
"Should" that is.
So if you take this route, just be very sure nothing expensive is in harm's way - just in case.