"In order to replace a thermosta in a heating or a cooling system, first of all, you need to start by shutting off the power to the heating or the cooling system. This will help you in protecting the HVAC unit. Secondly, if you are replacing a mechanical unit, you can remove the cover or the unit to get to the wire connections. In some of the units, you can do so by loosening two or three tiny screws while some of the units will have to be removed from the wall to have access to the wires and in case of the digital units they will just snap off the mounting base. An important step in thermostat replacement is to write down the terminals and the color of the wire. However, the thermostat replacement continues by mounting the new base on the wall. Last but not the least, you will have to check the functionality of the new thermostat by setting the power back to the heating or the cooling system. In case, even after following all these steps your thermostat does not start functioning you need to get in touch with an HVAC technician for better diagnosis. "