First of all make sure your thermostat is configured properly in Thinka (see Thermostat). Now just say: "Alexa, set the temperature to 21 degrees" or "Alexa, lower the temperature in the living room by 2 degrees".
In case something is not clear or not working according to your expectation you can contact us, and we will provide instant feedback. We can be reached by email at support@thinka.eu, or by phone at +31-20-2420666