I have to generalise here. If you close off both valves (usually by turning the knob or what looks like a screwdriver head clockwise) you can undo the big nuts (the ones closest to the radiator itself) turning them anticlockwise. At this point, whatever water is in the radiator will start to leak out. It probably will be pretty black, so have old towels or some container to catch the water. It will also leak out quicker if you open the bleed valve, at one end of the top of the radiator. You'll need a bleed valve key, readily available from DIY places. Once the radiator is emptied, it can be lifted off its brackets, up then away from the wall. After this I don't know what to suggest as it depends on what is fitted to your towel rail and if it is a close physical match to the size of your radiator. You could end up having to drain your whole system to replumb the pipes to suit. Hope this helps.