need to extract the .pff files
find the name of the crosshair file
make a new one in any image software that supports the filetype and give it the same file name as the original crosshair file.
either pack it back in the .pff or play the game with a .exe shortcut with " /d" at the end of its target (rt click > properties)
i'm sure if you use the search function you can find into on this. its been goneover before.
but unless this is for a MOD then i dissaprove. if you want to know why, then use the search again.