Easiest way, just cut the end of a PS/2 cable and crimp a 4p4c connector on the end, then you have a detachable PS/2 cable and don't really have to modify the keyboard at all. Otherwise, just open the casebody up take out the pcb desolder the wires that lead to the 4p4c connector and solder on PS/2 cable. There is really not much complexity there.