I cancelled "Delete a backup set and files" that was in progress. How do I now delete?

I have Norton Security Suite that was provided with my Xfinity internet service.  My backups are to an external USB hard drive (drive "K").  Drive K was nearly full after I created a new "backup set" and subsequently ran that backup.  I then selected the "Delete backup set" option and "delete a backup set and files" to remove the old set from K.  The delete process was progressing extremely slowly.  It probably would have taken a day or longer.  I clicked the "cancel" button.  The set is now gone from the "backup set name" selection box, so I have no way to restart the delete process.  How do I delete that set from the external drive?  It is named "{2C9FA39C-AA6D-4EEB-8BCC-E9C05DE4A61D}."  I am unable to just delete the folder via Windows.  Thank you.