According to research, the left as well as the right button must be pressed together with the OK button to get out of demo mode. The instructions can be found on the following link on page 7: https://cdn-vtech-jouets.vtech.com/assets/2e362e10-cb22-4890-b52b-17a1e6b7af33/PJ%20Masks%20Superlernuhr%20175804.pdf