There should be two metal access covers on the front.
The top one provides access to the top thermostat and element and the bottom one provides access to the bottom thermostat and element.
After removing the covers, you should be able to see the thermostat adjustment shaft and temperature indicator dial around the adjustment shaft. Typically there is an arrow like pointer which you would set to the temperature you want.
Unless you also have a tempering / mixing valve between the hot water outlet pipe and the cold water inlet pipe, you do not want to set this above 120° f for safety reasons (scalding and burning could result if the water temperature is set higher than that). If you do happen to have a tempering / mixing valve (they are rarely installed on residential water heaters), then you could maybe get a way with setting the temperature to 140° f (but never higher than 140° f in case the tempering valve fails and you get full temperature hot water from the tank), then set the outlet temperature of the tempering / mixing valve to 120° f.