The proper ratio is required to maintain the manufacturer's specifications for boiling point and freezing points, heat carrying capacity, prevention of scales and deposits and conditioning, lubrication and longevity of sealsin the cooling system.
The coolant can be flushed by draining the overflow tank, radiator, the heater core and the engine block according to the manufacturer's specified procedure, then running water throughout the system, then draining all that water, and then refilling the system with the proper ratio of new coolant and distilled or demineralized water. After this, the system must be purged of air and checked for leaks.