first, stewmac carry replacement lock-nuts, also i dont think your problem would lie in the nut, is your neck straight? are the frets in good condition? if your using a floyd (which is more then likely what you have) is it set up right? the nut is not usually the problem with floyd equiped guitars at least in my experience, and as for replacing it, unscrew the old one, and put the new one on, but make sure you get the right neck width.