If you are using a normal form, created by just setting the
action= attribute of your HTML form to
https://formspree.io/<your-email>, then you just have to replace the
<your-email> part, removing the old email address and inputting the new one. This is done on your site's HTML, without the need to set, tell or do anything on the Formspree side. You don't even have to tell us!
If you have a Formspree Gold account and are using a form generated from our dashboard, you must go to the dashboard, create a new form pointing to the new email address, then replace the path of the
action= attribute in your site's HTML with the code for the new form.