There are two main types of this valve:
They let air out of the system by themselves.
They usally cost more.
They are physically bigger.
Sometimes due to fair wear and tear there bodies stop sealing and they leak.
Isolate the radiator at both ends. (i.e turn off both valves)
Drain off some of the water in the radiator: either by loosening the valve to be replaced or slackening off a tail union. In both cases catching the water in a suitable container.
Slacken off (anti-clockwise) the valve your replacing, if water is stiill present, drain more off if not remove completely.
Screw in (clockwise) new valve tighten up everything you've loosened.Turn both valves back on and fill up by bleeding.
Check for leaks.
Simple to use.
Not much to go wrong.
You have to bleed yourself.
Losing the pin will cause it to leak.