public string PageNumberField {get; set;}

{
  //Modify the currently active map series and changes its sort field and page number field.
  //Note: Must be on QueuedTask.Run
  SpatialMapSeries SMS = layout.MapSeries as SpatialMapSeries; //cast as spatial map series for additional members
  SMS.SortField = "State_Name";
  SMS.SortAscending = true;
  SMS.PageNumberField = "PageNum";

  //Overwrite the current map series with these new settings
  layout.SetMapSeries(SMS);
}