Demo mode is a special firmware loaded onto watches intended for use in store displays. You might be able to install normal firmware by pushing the watch into recovery mode, pairing it with a phone and letting the Pebble app download the latest official firmware for your watch but this isn’t guaranteed to work as these watches weren’t intended for sale.
To get into recovery mode you need to hold the left, top-right and center-right buttons for about 45 seconds. The watch may reboot but do not let go of the buttons until you see a “loading” display. After a few seconds the “pair watch” screen should pop up at which point you need to pair it with your phone and Pebble app. The app should detect your watch and automatically download the latest official firmware.