Your thermostat uses three AAA (LR03) batteries. To change them, push the clip in figure 1, then open the case from left to right without letting go.
Then simply remove the batteries present in the thermostat, and put in a new set of batteries, respecting the polarity.
After you put in the new batteries, the thermostat should show you a tick "✓" indicating that it is working properly.
To check the battery level, simply go to your customer interface / mobile application, "My Residence" tab, then "My Products":