The username/password change is a core WP email, not one sent by Form or Connections. Connections, nor any of its addons allow or give the user a way to do this since it is built in to WordPress.
After a quick search I found this old plugin which might do the trick:
There does not seem to be many options available out there to customize this email.
Here’s a code snippet you might be able to customize to better suit your needs.
Hope this helps!