change opencart order email content as our wish

You can modify the content by editing the following template file:

/catalog/view/theme/default/template/mail/order.tpl

(assuming that you are using the default template)

But if you want to modify the subject and other stuff, you'll need to edit the order Model file:

/catalog/model/checkout/order.php

then find

$mail->setSubject($subject);

and set the $subject to whatever you want :)