Listbox items cannot be edited directly in the control: what you can do is to select a line an, transfer its content to a textbox control, edit text there and move text back to the listbox control. Available interactions with the listbox control are listed in the help.
An alternative to it is to use a tree customized to appear like a listbox: tree item labels can be edited directly in the control, at least those in the base column of the tree. You can look at the example \samples\userint\treelist.cws that shows how a tree can be customized this way. I haven't opened it, but this old example too should show the ability to edit tree cells.
