how to fix leaking radiator?

Hopefully you will have a valve at either end of the rad. One is the normal hand or thermostatic control valve . The other is what is called a 'lockshield valve'.
Close the hand control valve and take the plastic cover off the other valve. You will now need a small adjustable spanner or pliers to fit on the valve shank. Turn it clockwise until it is fully shut. With both valves shut you only have to contend with the water left in the radiator- the rest of the heating system can carry on as before.
Be aware that a thermostatic valve will only shut as far as it's frost setting. If the valve senses a freezing room temperature it will begin to open. So replace the faulty rad without delay.

One of Tony Blairs genuine Polish plumbers
January 2011