Answer: Set the new list count and list by XtSetArgs and install them by
XtSetValues.

XmString list[SIZE];
int list_size;

XtSetArg (args[n], XmNitemCount, list_size); n++;
XtSetArg (args[n], XmNitems, list); n++;
XtSetValues (w, args, n);


or similarly with XtVaSetValues:


XtVaSetValues (w,
    XmNitemCount, list_size,
    XmNitems, list,
    NULL);


Each time the list is reset by this the old contents are freed by the widget
and the new supplied list is copied.  Do *not* free the old list of items
yourself as this would result in the space being freed twice.  It is not
necessary to remove the items one at a time, nor to "zero" out the list first.