To return your game to the default settings, do the following:
- Open the Options window by pressing 'O'
- Click 'Reset All' button on the lower left
- Click 'Apply'
If you are unable to restore default setting in-game you can restore the setting by deleting the ClientOption.xml file located at C:\Program Files (x86)\Glyph\Games\Devilian\Live-US or Live-EU