How do I change the email content in Subscribe by Email?

Hello. I need to customize the content of the Subscription Confirmation message sent via Subscribe by Email plugin. I tried editing the subscribe-by-email/inc/confirmation-mail-template.php file but it made my site disappear. I have re-uploaded that file and the site is back, but now I'm afraid to mess things up again. Can you please help? Thank you!

Heather Weaver