How to Modify Output of a View

<?php
/**
 * Implements hook_views_pre_render().
 */
function jicustom_views_pre_render(&$view) {
  if($view->name=='stock_report'){
    $results = $view->result;
    foreach ($results as $key => $result) {
      $collection_name = $result->field_collection_item_field_name;
      if($collection_name == 'field_transaction_items_outgoing'){
        //Modify output
        $results[$key]->field_field_trans_item_quantity[0]['rendered']['#markup'] = -($result->field_field_trans_item_quantity[0]['raw']['value']);
      }
    }
  }
}
?>