In each shower you will find four knobs: the one on the top works to let the water out of the shower head, the second one for the smaller shower head, the third one for the bottom and the last one is the thermostat which regulates the hot water.
Important You need to be very careful before turning it because you need to press the button before and then, while you are keeping it pressed, you can gently turn it until you reach the temperature you desire.
