Removing the cover which has the "tado" logo embossed is the first step to getting into the thermostat.

It is secured with 3 clips.

One right next to the little opening for the temperature sensor .

Two other on the opposite side.

Once you removed the cover you can easily pry off the serrated ring. This serration is connected to a little cogwheel which transfers the turning motion to an electrical signal for setting the temperature.