Most hot tubs today use electronic or digital controls to maintain the water's temperature. To adjust the thermostat, you simply press the button that increases or decreases the water's temperature. Some hot tubs may use a mechanical thermostat to control the water temperature and you need access to the spa's heater to adjust this thermostat.
Turn off the power to your hot tub. You can do this at the breaker or GFCI.
Open the access panel to your hot tub's internal plumbing.
Remove the thermostat from the heater. This may pull off or require you to remove a few screws.
Turn the screw adjuster one quarter in the clockwise direction. This should increase the heat of your hot tub's water. To make the water cooler, turn the adjuster one quarter in the counter-clockwise direction.
Corroded thermostats will not respond to adjustments using a screwdriver. You will most likely need to replace the thermostat.
Do not increase the hot tub’s water temperature higher than 104 degrees Fahrenheit as this is normally an unsafe temperature.