This question should be moved to the help and trainning forum.  You go to the mission folder in the Hitech Creations folder on your C drive and delete the mission.  I used to keep a standard mission file out there and always load and modify that. Saved a little time, however, I noticed a problem with doing that so I deleted all mission folders and just one off each mission.  I would love to see a return of our old mission editor.  I felt it was a better tool.