Changing the Product Id to priority number

The current Opencart products gets sorted according to the product ID, I'm adding a priority number so the admin could set it.

Can anyone please tell me what are the codes to be replaced removing the product ID and replacing the priority number.

Thanks

Add sort_order to product in the database

Add database manipulation
/admin/model/catalog/product.php

Add sort order to interface
/admin/view/template/catalog/order_form.tpl

Then set up the front end to sort either with a filter or hard-coded database query.