Your hot tub thermostat will over time need to be replaced. The replacement is an easy fix but you will have to dismantle the spa heater housing to replace it. You will need your owner's manual to refer to so have it ready.
Step 1 - Getting Ready
Unplug your spa and turn off the breaker at the control box. This way you are doubly safe. Also, you should be wearing rubber-soled shoes so you are NOT grounded. Remove the cover from the spa motor. Keep all your screws in a bag for later. Locate the heater housing and remove the cover, placing the screws in the baggie also, or a separate bag if the screws are not all alike.
Step 2 - Removing the Bad Thermostat
Locate the thermostat. Disconnect the wiring on the end connected to the motor. You should see a capillary line going into a screw-in housing. There will be a lock nut holding the thermostat in place and you will need to use your wrench to loosen this. Your thermostat should simply pull right out of the holder.
Step 3 - Installing the New Thermostat
Slide the new thermostat into the holder and tighten the nut down over it. Reattach the new wires where the old ones were attached. Now put the cover back on the housing and tighten down the screws. Flip the breaker back on and plug in the spa. Turn on the motor and let it run to check and see if it is heating up. If so, put the door back on the spa, and, voila, you are done.