How do you fix the heater coil for a 2000 Chevy s10 blazer?

If the heater core is plugged you can try flushing it out. Disconnect the two heater hoses from the engine, connect a garden hose to either hose leading to the heater and gently turn on the water, gradually open water valve all the way until water runs clear then switch water hose to opposite heater hose and repeat. If the heater core is leaking and needs replacement I would strongly suggest purchasing a shop manual to walk you through the job of replacing the heater core. This is not a simple job.