You can fix the thermostat to the wall or place it on a flat surface (find instructions in the Installation section): where possible, place it at a comfortable height for use and away from heat sources or draughts to ensure that the temperature reading is consistent with the actual temperature.
If you are replacing your old wall thermostat with a Smart Thermostat, simply disconnect the latter while maintaining the existing connections, place your Smart Thermostat in position and proceed with the installation.
If you need to install a device that controls the ignition of the heat generator, you can also add the Starter, placing it next to the boiler, and install the Smart Thermostat in the area of the house where you want the temperature to be detected.