You need to remove the radiator cap for the coolant to drain. You may need to put the garden hose in the filler neck to pressure out the gunk if it was really dirty! The proper steps are:
1. Drain the old stuff
2. Fill with distilled water and bring to operating temperature with the heater on high
3. Drain the water
4. Fill with 50/50 mix of new coolant and distilled water
5. Run up to operating temperature and top off new coolant mix.
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