This is a common job for plumbers everywhere and I want to go over the basics of what to do to complete this job.Common indicators that you may need to change an element include a water heater that is slow to heat up or doesn’t heat at all.

First, you should determine if this is the cause of your problem.

To do that you should first be sure that the electricity is turned off of the appliance (You can do this by flipping a breaker, disconnecting a service disconnect, or flipping the switch of your timer). No matter where you turn off the power always check behind yourself with an electrical meter or non-contact voltage detector.
Then, remove the covers on the side of your water heater. They are usually held on with two screws, one on top and one on bottom. There will be insulation and a plastic guard behind these that also needs to be removed. Depending on the size of the heater there may be one or in most cases two covers normally located above and below each other on the same side of the heater.
Under each plastic guard there is an element and thermostat. The upper thermostat will look different since it has a high temperature cutoff and the reset button included with it. These thermostats basically read the temperature of the side of the tank they are pressed against and “tell” their corresponding elements to come on or go off based on if the temperature they read from the tank matches the temperature they are set to. They never come on at the same time so as not to draw to much current or electricity at one time. There corresponding elements are less complicated and heat up when electricity passes through them. Kind of like the filament in a light bulb but without the light. To test them remove one wire (either one) and test the resistance between both screws. It should read no more than 20 ohms of resistance. If more then the element is bad.

Next you should replace the element(s) that tested bad. Be sure not to skip any steps.

Again make sure the power is still off before you drain the tank. If the thermostat sends power to the elements while dry it can damage them or the water heater. You will also be disconnecting the wires to the element and that could pose a risk of shock if the power is still connected.
Turn off the water supply to the water heater. This valve is normally above the tank but if not you may need to turn it off at the next closest place possible.
Connect a garden hose to the drain valve at the bottom of the water heater.
Run the hose to the nearest and lowest available place to drain the tank. It will be draining through gravity so the tank will only drain to the highest point in the hose. The higher water in the tank will push the water through the hose until the water in the tank is at the same level as the highest place in the hose.
Open the hot water side of the faucet closest to the water heater. This will let air into the draining water heater and allow it to drain faster. A lot like venting an emptying gas can or bottle.
Once the water level is below the element that you are replacing turn the drain valve off and put up your hose.
Then, remove the wires from your bad element.
Use an element wrench or inch and a half socket to turn out the element. Most screw out though there are some that are fastened in other ways. Remember “righty tighty, lefty loosy”.
Once it’s out replace it with a matching replacement element ensuring that the gasket meets a smooth mating surface for a good seal.
Attach the wires to the new element. Order doesn’t matter just make sure they aren’t touching.
Fill the tank with water again and replace the covers.
Restore power to the appliance and enjoy a hot shower. You earned it