To relieve the pressure off a keg, all you need to do is get a big flat-head screwdriver and push the little stainless steel ball-valve down. That will allow the pressure to escape (along with a nice spew of stale beer).
Now if you want to take the whole valve assembly out (which I recommend, so you can clean out the keg), just follow the instructions on this webpage:
I hadn’t seen the updated version of that site. Good to see other people working out complex solutions to simple problems. The tabs he was hammering on with a screwdriver need a bung wrench (much like the socket tool that that the idiot made). The gold plated ones should run you about $15, an average one, about $7.
In a pinch, a set of pliers can be put into the cap, in this case with the nose of the pliers towards the ball in the valve, and opened as wide as possible and turned until the jaws of the pliers torque against the tab… or you can hammer on it with hammer and screwdriver or mill your own bung wrench.