The thermocouple is situated inside the burning chamber. It is held on by three screws that need to be removed. Because space is tight on the bottom side of the water heater, it is recommended that you remove the entire burner assembly and work on the thermocouple outside of the water heater body. After removing the three screws, pull the tube out and disconnect the wires, nothing which ports they connect to (red and white-colored wires). Install the new thermocouple, secure the screws and reattach the red and white leads. The entire process should take about an hour.