If the thermostat reset doesn't work to make it run the right temp, listen to the water heater as it heats up. Does the water heater make a boiling sound when it's running? If it's doing that, you've got huge amounts of sediment at the bottom and that can be helped for a little bit.

Turn the thermostat off and run the hot water until it runs cold. Then you can open the drain at the bottom to empty the water heater from the bottom. There's a garden hose fitting on it to run the water into a floor drain. Flush it with cold water a few times and the water at the bottom should come out full of crystals. Keep flushing until you stop getting crystals of sediment.

Close the valve. Refill the tank. Set the thermostat to "A" and let it heat up. If it runs hot now, that was it. It does mean that the water heater is on it's way out, but you can squeak another year or two out of it by draining out the crud.

The gas company in my area has service that basically puts a lifetime warranty on all appliances. Furnace, water heater, washer, dryer, fridge, whatever you want. It's a reasonable price and will save your ass in the winter. If my furnace goes out, I'll have a repair tech out to my house at any hour. They've come out at 1AM before. If you can get it, everyone should use a service like that. I've used the service many times on my terrible old appliances.