You will have to drain the system down to that radiator at least. Then, remove the existing valve as follows:
Obtain a Spud Wrench (second link below). IT is a flat-sided wrench designed exactly for this purpose and does an excellent job without damaging the fittings. There are other tools you can use including standard pipe-wrenches, but they tend to mark or damage the fittings.
Break the union on both feeds to the radiator - the ring that goes around the nipple that fits into the body of the radiator itself. In the picture, it is the piece of the fitting around the threaded brass part fo the valve. Once it is free on the valve part, you will be able to identify the same fitting on the other side. Remove it as well.
Shift the radiator out of line so you can get the spud wrench on the base of the valve. Here, it might be handy to have a standard pipe wrench nearby as well to restrain the riser pipe so you do not turn it as well, but typically the brass will not bind to the riser. Remove the valve entirely.
Now comes the complicated part - unless your replacement valve is exactly the same as your present valve in all dimensions it will not fit such that you simply drop it in place. So, take that valve body to your supply house and get the right fitting(s) to make it work. Typically you can get the union-nipple of various lengths and even increase the height of the riser. If you have to go lower, you will have to pull the riser nipple to do that - which often means working blind. But it isn't rocket-science - it just takes a little patience. I have done dozens without incident - although some required a second trip to the supply house to make it fit. Emphasis again: A Spud Wrench makes a PITA job FAR more tolerable For me, it was worth the investment for one (1) valve - little did I know at the time how much it would work into the future - but then, I also like good tools.
And, while you have the system dry, it might be an excellent time to invest in a few automatic bleeder valves for the higher radiators and consider adding a particle filter (not cheap, but a good idea for older systems) and even adding inhibitor to the system. Links below as well.How to change radiator valve?
Not much detail here but when changing any kind of valve you must first work your way up stream to a source where there will be another valve. This valve must be shut in order to isolate the section in which the repair will be done. Once you locate the upstream valve and close it the pressure in the piping down stream from that valve must be relieved . Only then can you replace the bad valve safely.
