You can buy Liqui moly radiator clean, pour it in the radiator. Turn the AC hot cold knob to hot. Let the thing run in your car for about half an hour. Run it for a whole day if you want. After that turn off the car, drain the coolant, remove the thermostat valve temporarily and flush the cooling system with plain water, when the water comes out looking clean, stop flushing. Use compressed air to drain out the water in the engine block.
Replace thermostat valve and pour coolant Liqui Moly, ZIC, Caltex, Suzuki (some coolants are pre-mixed and you don't need to add any water to them, some require mixing with water) follow instructions on the label. After pouring in the coolant in the radiator and overflow bottle, start the car. Leave the radiator cap open to allow trapped air to escape. Add more coolant if required. After car has warmed up, shut it down and put back the radiator cap. Turn the heater knob back to cold. Next day check coolant level again and add more coolant if required. You are done.