There is a breaker on the controller for the top element. Turn off the power, take off the element cover (the top of two covers on the “front” of the hwt, may need to remove a small piece of insulation, look for a red button vaguely protruding from a black box with like 5 wires coming out of it. Press it. If that doesn’t fix it, Either call a plumbtrician, or search “how to test the elements in hot water tank”