First - remove the source of the pressurization (the engines' compressors, or in a pressurized piston, an engine-driven pump). Shut down the engines ASAP and/or turn off the bleed air switch. Most planes leak enough to lose their pressure eventually if no new air is being pumped in. "Eventually" can mean 20-30 minutes to leak enough to get the doors open - but the bleed air can be switched off in flight, once below 10000 feet actual altitude, to give the leakage more time - IF one knows the valve is stuck ahead of time.
Second - Use the manual pressure dump controls in the cockpit (often a separate system/mechanism from the automated pressure control system: vacuum, hydraulics, electrics, magnetic solenoids, or muscle power).
Third - Some outflow valves can be accessed - when on the ground - from outside the aircraft. Ground crew can open them to dump the pressure.
Fourth - other means that professionals will know about - not for public consumption.
All of the above will vary depending on aircraft type.
In the case of SV163, one never knows. The crew did not shut down the engines ASAP - taxied for 2 minutes+ after landing and took 3+ minutes after landing to shut down the engines. They may have been overcome before being able to operate the backup pressure relief systems (if they even realized they had stuck outflow valves in addition to a fire). The ground response may not have known how to access the valves from the outside and open them (if the L-1011 valves were even externally accessable - specific model, 38 years ago).