The Eco mode cannot be selected while the robot is cleaning.
If your robot is cleaning, first you need to press the start button to pause the robot. The Eco button will then become active and you can select the mode by pushing the button Eco. Now press the start button again to run the robot vacuum cleaner in Eco mode.
If your robot is switched-off or in stand-by, you simply need to start it, choose Eco mode and then start the cleaning process.
All these steps can be performed either through Kobold Robot App or manually.