Override the Drupal Commerce cart summary using the theme preprocess function in either module or theme.

commercehook.module

<?php

/*
 * Implementation of hook_preprocess_commerce_line_item_summary
 *
*/

  // Replace the name of the hook with your theme or module name.
function HOOK_preprocess_commerce_line_item_summary(&$variables) {
  // Replace the label ( default to Total ) with your own label
  $variables['total_label'] = t('My New Label');
}

This is a code snippet for overriding defaults in Drupal commerce