Welcome to Android Central! Do you mean you're just trying to change the ringtone to something else, or are you actually trying to delete the ringtone completely from the phone? You can't do the latter, because preinstalled ringtones are saved to a protected system directory -- you'd have to root the phone to completely delete it.
If you're trying to change the default ringtone, are you using one of the other preinstalled ringtones, or trying to use a custom ringtone?
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