You just need to delete them from the folder they were in, and replace them with the new kill marks.
Remember to use the same file names And delete the current ones , Always make a back up of your original file. After deleting your current ones, Simply rename your new file and place it in this folder.
This is usually how to do it:
Go To Start
Go To My Computer
Go to ( C: )
Go to Program Files
Go To Z8games
Go To Crosffire
Go to Rez
Go to UI
Go to Mark
Make sure you restart your game if you were in game, or it will not take effect. I hope this was your question ^^