Installing a water heater thermostat for an electric water heater is not a difficult task. You will need to obtain replacement thermostats before you begin.
Are you sure that the thermostat is bad? Before you conclude that they need to be replaced you should test them. See the article 'Testing Water Heater Thermostats' for more information on how to conduct a test. A common problem that may seem like the thermostat is a bad heating element. See the article 'Testing Electric Water Heater Elements' for more information.
Not quite sure what your problem is? See the article 'Troubleshooting Electric Water Heaters' for a complete discussion on all of the water heater problems.
Electric Water Heater Thermostat Function
The thermostats for an electric water heater serve two important functions. First they are used to set the teperature for the hot water in your tank. Most of the time you should stay with the factory settings. See the article 'Hot Water Heater Temperature Settings' for some guidelines and a discussion on the dangers of setting the temperature too high.
The second function is to sense when the water has cooled to the point where the elements need to be turned on to heat the water in the tank. Most hot water heaters only run one element at a time. So the upper element will need to get the water up to temperature before the lower one will engage. This method reduces the amp load that the water heater draws. The down side is that it takes longer to heat a full tank of cold water.
WARNING!! DO NOT WORK ON A WATER HEATER WITHOUT TURNING THE POWER OFF!!! WARNING!!!
Purchasing an Electric Water Heater Thermostat
The fist thing to remember is that most water heaters have two thermostats, an up and a lower. The upper one is the larger and more complicated of the two. The upper thermostat controls the lower one.
If you are going to the trouble of replacing them, you should replace them both.
Removing the Old Thermostat
There are two steps to removing the thermostat assembly. First you need to unhook the wires. Second you need to release the pressure on the spring clips that hold the thermostat in place.
Removing the Wires
The upper thermostat will have more wires attached to it. The reason for that is that the upper thermostat also controls the lower thermostat.
Installing the Thermostat
Slide the new thermostat behind the spring clips. Make sure it is snug. A loose thermostat will not sense the temperature correctly.
Do you have that picture? Are your wires tagged? Then hooking the wires back up is a snap.
Hook up the wires per the original connections. Replace the plastic cover and insulation. Then install the metal covers. At this point it is safe to turn the power back on. You should be getting the proper of level of hot water pretty soon.