Below, a few of the Master Appliantologists here at the Academy offer some tips and procedures for repalcing the heating element with a tank full and not making a flooded mess:
Quick and dirty water heater element change
turn off power to heater
turn off water to heater or house
re-leave pressure anywhere you want but when it's gone close the valve (sink , tub ect )
remove cover and disconnect wires
get new heater element ready , remove from package , put on gasket ect
put big towel down in front of heater
loosen old element with wrench about a half turn
hold new element in one hand , unscrew old element with the other , and switch them out quickly ( make sure as your unscrewing the old element the o ring comes with it )
use wrench to tighen , hook up wires ect , your done
if done right you will loose less than a glass of water on the floor
always leave yourself the out if things go wrong of reversing and putting the old element back in , if you can't get the threads to start or something like that
I have done hundreds this way and even the old 4 bolt type with no problems at all
I change all my elements without draining.
You lose almost no water.
However,keep a towel and shop vac close by.
The only problem comes in when it is a old corroded element and the rubber gasket wants to stick in the hole.
Other times,the element is busted and hangs up coming out.
If the gasket sticks,remove the new gasket from the replacement element and re-use the old gasket.
NOTE: You can not drain a heater from the high temp pop valve.
I change the element with out draining the tank... Turn off the power........Shut off incoming supply line........Open the hot side of the faucet to relieve the pressure then close the valve..............Loosen the element a few turns and you should be able to see if the gasket is stuck to the tank, just use a small flat bladed screwdriver to pry loose....... have the new element in hand as you remove the old one........ when removed, just drop it and insert the new one......you are lucky if the customer has their tank sitting in a drip pan...... ..........If you need to remove the water, disconnect the incoming line and the outlet side..................................Use some compressed air on the outlet side ( draw tube high in tank )...... water level will be expelled out the incoming side to the level of the fill tube ( lowest point )..... Not all of the water will be out , BUT, it will be lighter.......
Source: suggestion for new electric water heater?