Check a parts store for a recommended name brand flush. Follow thw directions on the can/bottle. Then flush out the radiator in reverse (use a rag on the end of a water hose and stick it into the bottom opening in the radiator. Have the top hose off, then run the water to "back-flush the radiator. I would pull off the thermostat housing and do thw same thing to the engine.
Be sure to use a new thermostat, gasket, upper and lower hoses, and cap. I am having great success running a 60/40 mix of antifreeze. (60 is antifreeze).