Add A String Item, Returned From Another Form To A List Box At A Specific Spot In Place Of Another?

there's a list-box with items, and say you want to modify an item in the middle of the list. You select that item and click "Modify" button and a new form appears with the previously selected item data from first form ready to be modified in a text-box. After modifying and clicking Ok the second form suppose to return that modified string to the first form and insert the modified string into the same spot instead of the originally selected item, so it looks like it was edited to the user.