How do I change the auto-reply to the submitted forms?

You're most likely referring to the emails sent to your customers/visitors after form submission. We call these AUTORESPONDERS and you can freely edit the email template for this by following the guide below.

Complete guide: How-to-Edit-the-Email-Template-for-Notifications-and-Autoresponders

The same idea also applies to the emails you receive as the owner (NOTIFICATIONS).