Water Heater

You adjust the thermostat's temperature dial ...generally located at the bottom of the tank.
http://www.houselogic.com/home-advice/water-heaters/water-heater-energy-saving-tips/#.

What Ragman said, it's usually red.

Turn Down the Tank’s Thermostat

"For every 10 degrees you turn it down, you’ll save 3% to 5% on your bill. Most water heaters come preset at 140 degrees, which has the added risk of scalding. The Energy Department recommends most households lower it to 120 degrees. That’s high enough for your needs, and high enough to reduce mineral buildup in your tank and pipes.

Here’s how to ensure you get 120 degrees:
First measure to see what temperature your water is at now. Don’t trust the thermostat. They are often inaccurate. Instead, use a thermometer to measure the temperature of the hot water at the faucet farthest away from the heater.

To remember this setting, mark that temperature on your thermostat.
Now turn down the thermostat to what you think will be 120 degrees, based on your earlier measurement.

Wait at least 2 hours. Measure the water temperature again at the same far-away faucet. It may take a few attempts to get it right.
Once it’s right, mark that spot on your thermostat so you’ll remember it.

If the thermostat on your water heater doesn’t have a numbered gauge, put it midway between the “low” and “medium” marks. Wait a day, and then measure the tap temperature as described above. Keep adjusting until you hit your target temperature.

Keep in mind that some water heaters have two thermostats — one for the bottom heating element and one for the top."

Many manufacturers set water heater thermostats at 140ºF, most families usually only require them to be set at 120ºF, which also slows mineral accumulation and oxidization in your water heater and pipes. Water heated at 140ºF also stances a safety hazard.