If you want to edit your form, it is possible, though the method isn’t very intuitive.
While you’re editing your post or page, just place the text cursor anywhere within the form code, and click again on the “Add Contact Form” button. It will bring up the popup for that same form. Make your changes, then click the “Add this form to my post” button again. It will not add a new form, but will apply your changes to the existing form.
via: https://www.wpsuperstars.net/jetpack-contact-forms/
