Unless the shutoff valve is leaking, there's no reason to drain the boiler. Shut down the boiler, let it cool, then manually lift the relief. If the shutoff is holding, you should get little to no water coming out. With a wrench, unscrew the old valve and screw on a new one.
The ASME Boiler and Pressure Vessel Code is quite strict in its requirements, so be sure you purchase exactly the same valve.
The ASME Boiler and Pressure Vessel Code is quite strict in its requirements, so be sure you purchase exactly the same valve.