Each 10 degree reduction in your water heater’s thermostat setting will save you 3-5% of your water heating costs (source: American Council for an Energy-Efficient Economy). For a standard tank type electric water heater, that’s on the order of 200 kWh, or roughly $30, per year. You should set the temperature at the lowest setting that keeps everyone happy (particularly the hottest showerer in the house). The analog knobs (there are two, one for each of the heating elements) for setting the temperature can be found by unscrewing the two plates (one low, one high) on the outside of the tank. Once you’re in, a little trial-and-error may be necessary – set the temperatures down some and then wait a day to assess the hot water’s warmth; then adjust some more, if you’re still mixing a lot of cold into showers.