Doing a search in the Tools ... Developer's Tool Kit ... for the words:
Order Update
Would locate the use of this in:
/admin/includes/languages/english/orders.php
Line #75 : define('EMAIL_TEXT_SUBJECT', 'Order Update');
A further search on:
EMAIL_TEXT_SUBJECT
Would show that this is used in:
/admin/orders.php
Line #124 : zen_mail($check_status->fields['customers_name'], $check_status->fields['customers_email_address'], EMAIL_TEXT_SUBJECT . ' #' . $oID, $message, STORE_NAME, EMAIL_FROM, $html_msg, 'order_status');
Line #129 : zen_mail('', SEND_EXTRA_ORDERS_STATUS_ADMIN_EMAILS_TO, SEND_EXTRA_ORDERS_STATUS_ADMIN_EMAILS_TO_SUBJECT . ' ' . EMAIL_TEXT_SUBJECT . ' #' . $oID, $message, STORE_NAME, EMAIL_FROM, $html_msg, 'order_status_extra');
Note: there is no override for the file ... so keep a backup of the original and your changed file for future updates ...