what specifically are you having difficulty with?
have you got the Dynamic Drop Down working on the insert page?
it's the same setup on the update page, except that the Set Selected in List for the parent list, populate list from array and set selected in list for the child list behaviors need to be added to the body onLoad event.
on the onLoad event, these behaviors will change the selected item in the parent list, then populate the child list and set the selected item in the child list.