1) How overcarb'd it is.
2) How soon you need it drinkable.
A "tame method for knocking out CO2 is to loosen the lid and set it ajar for 12 hours at a time...testing it along the way. If the keg is cold, it will take much longer for the CO2 to escape solution.
A more aggressive manner, one which I've used, is to remove the lid, stick in a sanitized paddle and give it a very gentle stir until you see foaming. Think of it like stirring a glass of beer.
If the keg is severely overcarb'd, be very careful with this method. Removing all of your kegs from the kegerator and mopping up 2 gallons of foamed over beer is no fun.
Let is warm up some to speed the release of CO2. Kept cold it will take longer than you might expect. Pull the pressure relief valve anytime you are near it. Sample it occasionally and when it's about where you want it, chill it down and put the gas to it at serving pressure.