The thermostat regulates cooling water flow, opening and closing depending on engine heat, allowing more or less water to pass, thus keeping engine temeperature stable. Thermostats cannot be repaired.
An engine should be run at its designated operating temperature for best performance. The thermostat keeps the cooling water temperature in the correct range, sensing the water temperature in the engine and responding by opening or closing, allowing water to circulate when the temperature rises, and preventing circulation when the temperature falls. The thermostat is basically a cylinder filled with wax which expands as the temperature rises. At a set point the valve opens to allow flow into the engine water cooling passages. A hole in the thermostat bypasses the valve and allows a small amount of flow at all times.
The opening temperature of a thermostat is normally marked on it. For raw-water cooled engines this will be between 45-50°C, for indirect cooled engines 79-85°C.
A thermostat can be tested by immersing it in hot water, and checking the temperature at which the valve opens against the temperature of the water as shown by a thermometer.
It is not possible to adjust or repair a thermostat. If it doesn’t work properly it needs to be replaced. If it is jammed closed and the engine is overheating it can be removed and the engine run without it, albeit cooler than its designed temperature.
It may not be obvious in which direction a thermostat works, so check the service manual. If it is put in the wrong way round it will not open and the engine will overheat.