Buy a flush kit from you local autoparts/hardware/walmart and follow the directions.
Be sure to have a large container to collect the old coolant. Dispose of it at your local gas station.
It's pretty simple really. Open the fill cap, open the drain-****, let old coolant drain as much as possible without adding new water (you can even start the car to make it drain faster...won't hurt it as long as you are watching your temp gauge/light and listening for water pump noise). Once stopped draining, flush with fresh water. Let drain. Once stopped draining, flush with fresh water, fill up, start car, let circulate until clear out of drain, let drain completely, close drain ****, fill with selected coolant and DI/RO/Distilled water mix, close fill cap. Start car, let circulate to remove air bubbles, stop car. Open fill cap, check level, fill again as necessary.
Many coolant fluids are poisonous, and if glycol based, smells and tastes sweet to animals and children, so be sure to dispose of it properly.