By shutting off the water supply and replacing the valve
Mixing valve usually for tempering domestic hot water.
There are no pressure valves on a regular home hot water system. If you are meaning some kind of industrial application,then say what exactly it is.
Connect the domestic coil thru a tempering valve to the hot water pipes in your house and turn off the valves to your old hot water heater. You should include a shutoff valve so you can revert to the hot water heater when you have your coal boiler shut down.
Isolate it from the water supply then replace the defective valve
You turn off the water valve before it -probably on the sidewalk, or just on your front yard. Then remove the main valve and replace it. NOT a job for amateurs. I hope you have some experience at least.
Open a vent valve to purge the air and open the supply valve to let in water. Every hydronic (forced hot water) system has a system fill or supply valve, often connected to domestic cold water through a flow-check valve and a pressure-reducing valve. However, simply opening this valve with the rest of the system closed will merely pressurize the system with air. The system should also have one or more automatic air vents, plus a purging valve or two. By allowing air to vent (often out the top) and air bubbles to pour out through the purge valves, you can mostly fill the hydronic system with water. As the system is then heated, more air bubbles will form from dissolved air and be vented by the automatic vents. If it is not automatically vented, the top-most loop of the system may accumulate air and stop working until it is purged by opening a vent until water comes out.
When the pressure becomes erratic time to replace
A valve to bleed out air manually
First check to see if the valve for the water supply is turned on. If that isn't the problem you may need to replace the tank water valve, they do wear out and are not that expensive or difficult to replace.
no water should be coming out of the safety valve , only if water over a certain temp orthan creates a pressure if water coming out and water not steaming than replace valve
Normally I just replace it
A 1999 Dodge Ram does not have a water control valve.