Boiler ? Suggestion to replace Relief Valve -- Do I need to drain boiler First?

Yes, you must turn off power and drain the boiler to change the relief valve. You need only drain it enough to remove the relief valve without loss of water. Be sure to vent off the air allowed to enter when you restore power. There will be a valve somewhere on the water feed line which will allow you to turn off the incoming water. There will also be at least one drain valve and one or more vents. Vent from the highest point to get as much air out as possible. Your system should have some automatic air vents built into it. That will take care of any air remaining. Be sure your replacement valve is the same as the one taken off. It MUST be the same size and it MUST match the relief capability of the one being replaced.