Hello,
If you're referring to the confirmation email sent by AcyMailing when a new subscriber is created, you can edit the email in the AcyMailing configuration page, tab "Subscription".
If you're referring to the registration email sent by Joomla when a new user account is created, you can customise it in AcyMailing under the "Newsletters => Joomla notifications" menu (available in AcyMailing Essential and higher)