Find the Heater Contactor in the control box (see picture below).
If the heater contactor it is not arced/welded shut and moves up and down freely, then the thermostat needs to be replaced. Thermostats are located in one of two places on a spa, either on the top of the spa (spa sides), or in a control box where the equipment is. See Thermostats in Spa Parts section. Replace the contactor if it was arced/welded shut. See contactors in Spa Parts section. In some cases both of these will need to be replaced.
NOTE: Contactors arc or weld themselves shut due to "chattering" caused by:
ONE MORE NOTE: Leaving your jets (jet pump) on for a long period of time will also cause the water to over heat.