Goto woocommerce settings -> emails

There are 4 emails you can customise for Order Approval Plugin.

1. New order notification to admin
2. New order notification to customer
3. Order approval notification to customer
4. Order reject notification to customer

To override and edit this email template copy order-approval-woocommerce/templates/emails/wc-customer-order-approved.php to your theme folder: yourtheme/woocommerce/emails/wc-customer-order-approved.php.