The coolant should have been drained every 2-2.5 years. Then refilled with DexCool to be compatible with what was left in there.
In your position, I would drain the coolant using the drain on the bottom of the radiator (there's a hole in the air deflector underneath to allow you to put a tool up to turn the plastic spigot to drain. Clean off underside of radiator cap with toothbrush and dish soap (an old one of course!)
I would refill with water and run to operating temperature to get full circulation. Then I would drain again, cool, and refill. Then repeat once more. I would refill with DexCool of the proper amount for the volume cooling system you have then add water to top off. Run to operating temperature and refill radiator after cooling. Then next time at operating temperature (lower radiator hose warm from water circulating through radiator) run engine at 2500 rpms for 10 seconds or so and repeat 5 times to remove air bubble from heater core. Let car cool so you can take off radiator cap. Have someone run motor after it idles to operating temp at 1500-2000 while you fill radiator. When engine speeds up the level in radiator will drop and you can add more coolant. Then while motor is still at speed, put radiator cap on. Then open bleed screw on top of thermostat housing to let air out. Fill reservoir.
I would repeat in one year since coolant hadn't been changed in timely manner.