The answer to 'how to unclog water heater installation' is to first turn off the power to the water heater and shut off the cold water supply. Then, drain the tank by connecting a hose to the drain valve at the bottom of the tank and running the water until the tank is empty. Next, use a plumber's snake or auger to clear any blockages in the tank or the inlet and outlet pipes. Finally, refill the tank and turn the power back on.
It's important to take safety precautions when working with a water heater, as the water can be very hot and the tank is under pressure. If you're not comfortable performing this task yourself, it's best to call a professional plumber to handle the job.