How to delete the entire library

Delete MyVideos34.db from XBMC\userdata\Database.

You could also go to Settings > Video and clean the library. That will remove any items in the library that it can no longer find on the disk, then just rescan.

I recently migrated my XBMC library from one server to another and just changed the locations in the database, but I was using MySQL, so I'm not sure how I would do the same thing in SQLite, which is the built-in database.