The original volt 30 amp hot water 10 gauge circuit wires will need to be verified that they are not being used for some other purpose. Then metal flex is installed between the junction box and the water heater. Did you read the beginning of this article? Top and side connections. When the lower thermostat acts up you will have hot water, just not very much.
The upper and possibly the lower thermostats will have a red reset button.
Electric Water Heater Wiring
I wired the water heater just like the old one was wired, with the white wire connected to the ground screw. Any hotter than that and you run the risk of serious burns. The trade off it that it uses more amps and requires heavier wire. How do I wire it? Wait for full stream of water. You have both skill issues and safety issues.