On most gas water heaters the thermostat is usually visible. Setting the temperature is as easy as turning the dial to the LOW or WARM setting.
Some gas heaters have the thermostat protected behind a panel that can be rotated open.
Flip this panel open and turn the dial to the appropriate setting. Electrical water heaters generally put the thermostat behind a screw-on plates to protect people from electric shock. Before removing this plate by loosening its screws, shut off the current to the water heater by unplugging it. Adjust the water temperature, and then replace the plate before plugging the water heater back in.