How to undo an union on sacrificial anode?
What you are assuming is the anode rod appears to be your hot water outlet. The anode rod should be under that plastic cap you see in the pic toward the back of the water heater. Removing the plastic cap should expose the top of the anode rod which will require a   1 1/16&quot; socket wrench to remove.
Before you do this be sure to close the water inlet into the water heater shut down the heating element and depressurize the tank. When reinstalling the anode you can use pipe thread dope or teflon tape on the threads.