I'm going to go out on a limb here, but if you mean the Water Heater Reset button, they are usually on the Water Heater, behind the outside access panel. Should be a black "box" with a round rubber plug. However, I have found that if you just turn the 12V switch off and back on again, it will do the same as the reset button.
If you are looking for some other reset button, please be more specific!
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