If you let your car set long enough. The pressure may drop on it's own.
Or use a hose with something on the end of it to push the valve in under the schrader cap.
Drain the gas through the hose into a container.
That should relieve the pressure.
If you are really worried about it..just unplug the fuel pump connector at the tank and start the engine to suck up the fuel....put a shop rag under the line and loosen the connector as salvage said..the rag will catch the dribble...normally, if the car has sat an hour, most of the pressure will have bled down and I never bother relieving it....I just use the rag, if I remember.