The thermostat on a Reliance 501 electric water heater is usually located in a metal box on the side of the tank, while the thermostat on gas water heaters is usually found near the firebox. Some units also have an external thermostat mounted on the wall near the unit.
Consult the unit's instruction manual for help locating the thermostat. Often, thermostats are protected by a metal box or other covering that needs to be removed before the temperature on the water heater can be adjusted. Most thermostat covers can be removed with a standard screwdriver or pair of plies. Always turn off the electricity on an electric heater before accessing the thermostat.