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Is this an all electric cylinder or does it heat via a boiler aswell? The Electric thermostats are pre-set to 65 degrees C which is a good guideline to the temperature required to combat any legionella forming in the cylinder. You can as far as I am aware just turn the red dial down but unless you use a thermometer there is no way to judge temperature. If the cylinder is being heated via a boiler there will be another thermostat on the cylinder which you can adjust.
There should be a timer on the wall somewhere this will have hot water and heating on it