If a water heater has a screw-in electric heating element, it requires a water heating element wrench to change it. This wrench is a large socket that snaps onto the end of a ratchet to remove the element.
Some water heaters have a flange style element that requires no special tools but use four Phillips head screws that are removable with a Phillips screwdriver. The screw-in element is easily distinguished by the large nut where it meets the tank. To access the element, remove the metal cover from the side of the tank and push back the insulation. It is sometimes necessary to remove the thermostat before removing the element.