The correct and safe way to reset your preferences is the following:
- Open Terminal.app
- Run: defaults delete com.jamawkinaw.downcast.mac
If you're reading this article because you already deleted your file manually and your preferences won't stick following a restart of Downcast you will need to log out and restart your Mac to recover. You'll know you are in this state if you find that your preferences reset each time you restart Downcast.