If you are lucky you will not have to drain down the system,shut off both end valves if you are lucky they both will shut sometimes they let by,again if you are lucky there will be enough play in the pipe work to lift the rad from the wall brackets,most times there is not,if there is follow the other posts and slacken just a little the two large nuts and tilt rad forward to get to the faulty bracket,there will be some leak so putsome towels down and a couple of containers,if there is not enough play to lift rad you will have to drain that rad by slowly letting the water out from one of the nuts you have loosened,depending on how big the rad is you should get about a two bucket fulls if the valves have shut off if not you will be draining the system via that rad