Those expecting parts from me...

Disconnect and ohm the element.

They are usually OK, or open, although I have seen them out of spec, but rarely.

It is usually The bottom one, as stated.

After checking both, reconnect it and turn up the bottom about 7 degrees, then turn up the top one 12*. Notice how the pots feel, if they feel bad, they might be.

Verify 240 at the bottom element, then get yourself a beer, sit down in front of the unit, and wait for the click, as the thermostat switches to the top one, then verify 240 there.

That's pretty much it.

If it is the bottom element, get a 16 oz cup, and a bath towel (folded and bunched below the element), and you should be able to change it without losing a full 16 oz of water.

In my experience, you are actually more likely to ruin the drain valve (and thus the whole thing) trying to drain an old unit, than messing up anything else.

(put the rubber seal on the new element before loosening the bolts, or element, if it's screw in, and have everything oriented and ready to bolt back so you save those few seconds)

If the pipes are in good shape, and you don't open any faucets open, you should have enough vacuum to create a relative hydro-lock to help prevent loosing water.

For as simple as they are, water heaters are awfully intimidating for a lot of people.
Even professional building maintenance folks, for some reason.