In order to remove your current progress and start over on Android:
1. Tap on "Disconnect" (the left one of the two buttons) to disconnect your Google Play account.
2. Delete data in Settings – Apps – LDoE.
3. Restart the app and connect it to Google Play for your progress to be automatically saved.
In order to start the game over on iOS, you just have to disconnect your game progress in game settings and reinstall the game. Please note that pressing "Connect" (still the left one of the two) button should not lead you to restoring some other progress, if this is what happens, you should load this other progress first, only then do the previous steps.