In order to start make sure you drain the cooling system, then you should disconnect the top hose that connects the radiator header tank and remove pre heather housing. Subsequently, release the lower hose from the radiator outlet pipe and remove the screw bolts that fix the radiator to the front engine compartment mounting panel. Lever the radiator out and be gentle on the cooling fins and also avoid spilling of the antifreeze solution on the bodywork. Radiators, particularly those that are complex should be repaired by a specialist while simpler leaks can be arrested with a specific compound. The foul smelling flies should be rinsed off the radiator matrix with a soft brush or water hose and the matrix should be flushed in proportion to the amount of contamination. Any hoses or clips which may have corroded should be replaced and the drain tap and washer should also be replaced if needed. Last of all reattach the radiator the opposite of how you removed it, refill it then check if there are leaks.