Are the results of each mission are already saved permanently or you can clear something like the kind of save in the game "Mark of the Ninja"?
As far as I know, the files are stored in the Steam cloud (if you are playing with Steam). I found a good guide: http://forums.steampowered.com/forums/showthread.php?t=2470784 on how to delete those files.
If you are not using Steam, then just reinstall the game or replay the mission to change the results.
It would help, if you could give details on why you want to delete the files