The email text in v1.3.x is built by combining several language components together at the time the email is sent.
The fastest way to find the file where the text is contained is to select a few words from the content you want to change and do a search for them using the Developers Toolkit in the admin. https://www.zen-cart.com/tutorials/index.php?article=38 and https://www.zen-cart.com/tutorials/index.php?article=39
Then use your FTP program to download the appropriate file, make edits to the file using a text editor such as notepad++ and then upload the changed file back to the server.

Ideally, if you're going to be making several changes, you would also be sure to create an override folder for your customized template and stuff your edited file into that folder instead of overwriting the original. You can see more about adding/editing templates in the FAQ area, as well as discussions on how override folders work.