drain the radiator and measure how much came out, now open the thermostat housing to remove thermostat valve, push a blower nozzle in it and start the blower, it will push out all dirty water from the engine, measure what came out again. Write it down.
Put thermostat valve back, replace drain valve on radiator, add pure coolant to the engine, half of the quantity that came out, start engine and slowly add water, keep on adding until no more would go in, do not let it overflow, coolant is poisonous. Keep the engine running till the radiator fan starts, squeeze the rubber pipes to rid of any stuck air bubbles in the system. Once the system is stablized, put the cap back on, and observe any anomaly.