Never do anything to the radiator until it has cooled down. Look for the reservoir tank (this is mostly a plastic tank), find out your drain valve that’s usually at the bottom of the unit and ensure to put the drain pan under it before opening the drain. Remove the lower radiator hose and move the hose clamp from the radiator neck and insert a screwdriver between the hose end and radiator neck to empty the hose and then twist it to disconnect it. The process of draining the radiator removes 50% of the coolant.  Insert the garden hose into the top radiator connector, insert the radiator pressure cap, and start water flow. Flush the radiator until the water is clear to your eyes.