The code to change the quantity label in a Drupal Commerce product

gistfile1.txt

 * Implements hook_form_alter().
 *
 * Modify quantity text
 */
function HOOKNAME_form_alter(&$form, &$form_state, $form_id) {
  if (strstr($form_id, 'commerce_cart_add_to_cart_form')) {

    // Rename add-to-cart form fields label (quantity).
    $form['quantity']['#title'] = t('Quantity');
  }
}