My husband drained it without success. There is nothing you can do about extending the heater's life because it's dead. Somehow, water has rusted metal parts, and the rust is getting into the water. The ony cure is replacement. It's too bad about that 5-year warranty, but it's a fact of life.
Heaters usually go, it seems, just a few months after the warranty expires.
Maybe you can do better with a heater that has a 10-year warranty. It works OK, but I hear sloshing of water in the expansion tank. The tank has a diaphragm. Is there a way I can stop that sloshing. You can recharge the tank.