Turn off the gas to your Heatilator fireplace. Wait about one-half hour for things to cool down.
Open the maintenance hatch below or to the side of your fireplace.
Locate the pilot light inside the hatch. The long metal stick next to the pilot is the thermocouple.
Disconnect the thermocouple line from the gas line with a wrench.
Remove the old thermocouple and secure the new one in its place.
Close the maintenance hatch, turn on the gas and turn on the fireplace.