I will tell you how I do it because I don't understand your problem :
I have 5 waypoints (01, 02, 03, 04, 05).
I want to edit the second one so I do PREP (NOT DEST) 02.
I set the parameter selector on L/G and I see the current waypoint (02) coordinates.
I press 1 or 7 to edit the left part then INS and 3 or 9 to edit the right part.
Done.
Have you tried to read the INS part of the manual ? It might be easier to find the answer to your problem here.