Interval depends on vehicle and coolant type. Some vehicles have long life coolants.

You can have it flushed at the dealer or at most gasoline stations or even DIY. Just dispose of the used coolant properly as it is very poisonous [yet very tastes like Kool-Aid].

On some engines, you also have to open the drain in the engine block (aside from the radiator drain itself). Kasi kapag closed ang thermostat, hindi mo made-drain lahat through the bottom drain.