To delete the Wi-Fi credentials, simply touch the two arrows below the display for 2 seconds - this even works when the arrows are not lit.
Note: Please make sure to position your fingertips only on the arrows. If the display is touched the procedure will fail.
At success the service menu will be opened, which is indicated by a wrench and screwdriver symbol.
Use the arrow buttons to cycle through the menu until you are at the menu item showing a Wi-Fi symbol and a trash can.
Touch the main area of the display to confirm deletion of the Wi-Fi credentials. Blinking of the Wi-Fi Symbol and trash can confirms the deletion.
After deleting your Wi-Fi credentials, you can enter new ones using the tado° app.