If there is a hand shutoff valve on the radiator that still works, shut off the radiator valve, open the connection slightly and drain the water into some pans and a bucket. If there are no shutoff valves, turn the boiler power switch off, shut off the feed water valve near the boiler, start draining the boiler, go upstairs to the top radiators and open the air vent valves. drain the system below the radiator to be removed.