What does coolant do?
Coolant, also called radiator fluid, it keeps your engine temperatures stable under all climate extremes and driving conditions. Because your engine can produce high level of heat when it is running and the fluid flowing through the radiator system can help to draw some of the heat away from your engine so it doesn't over heat and it also prevents the water in the radiator from freezing under colder weather.
Generally you should complete your coolant flush and change for every 24,000 miles or two years because there is a life span of chemicals in the radiator fluid. Follow your factory service manual or maintenance manual for better radiator fluid change procedures.
Watch the video below on how to change your car's radiator fluid as a reference.