Why has nobody mentioned this:
Turn OFF the water feed into the tank. The feed in will be at the bottom of the tank, so locate the pipe entering the tank at the bottom and follow it until you find an isolating valve, usually a gate valve in older installations. If no valve then it's a poor install and you will have to tie up the ball c ock in the header tank and drain that as well (which will take a lot longer)
There should be a drain point at the bottom of the tank. Sometimes on the same entry as the feed in, sometimes on a separate port. Attach your hose to that and run the hose straight down out of a door into the garden. Make sure the hose is nice and straight with no kinks otherwise it will drain very slowly or not at all. Open the drain valve and wait.
You don't need to drain the whole tank, just enough to clear the heater.
Remove heater with spanner.
No sealant needed when re fitting, just the big fibre washer supplied with the heater. Make sure you clean all the remnants of the old washer off the flange so it's nice and smooth and clean.
If it weeps and won't seal (damaged flange), I use Hawk Potable water jointing compound.
Don't forget to charge "plumbers rates" for the plumbing bit of the job, as up here plumbers rates are higher than sparky rates.
Don't forget to take some rags to mop up the inevitable spills.
And keep an eye on the water level while the heater is out. I had one where the stop c ock didn't turn off fully and the tank started to slowly re fill. I had to leave the drain valve slightly open all the time I was working to prevent it filling up.
