I believe it varies, but I just did mine a couple of weeks ago, so I'll tell you how it was with mine. There was a plain curved metal plate over the thermostat - No indication of what was under there. It was held on by a single screw. Once I took that off there was a small dial with a slot (like a screw head) that you could turn with a small coin or the tip of a butter knife.

One thing to watch out for is that it may take a long time to register the change. The cylinder will be very well insulated. Depending on how much hot water you use, it may be the next day before you notice the difference. So change it just a little at a time.

As for 120 deg F, I'm not sure that's advisable on hygiene grounds - Here they recommend at least 55 degC, which is about 131 degF, to kill the bugs that would otherwise breed in your heater.

For turning it off, there should be a switch, like a normal light switch, somewhere in the cabinet - just look around in there with a torch - it may be awkward to access. (My cat's so cute - I just opened the door to the hot water cupboard to have a look, and she decided that was an ideal place to explore).

Anyway, if you turn it off, you'll have hot water for a while, but then after you turn it back on, it'll take a long time to reheat, and you may be short when you really need it. I suppose you could experiment - if you don't mind the risk of a cold shower.

John.