If you are working in a Classic Signup form, head over to your Audience page and click on the Classic forms tab. Click on the signup form that you would like to edit. Once the form is open click on the Confirm it tab and make any edits to the confirm opt in email that is sent to each new contact when they signup to join your list.
That should do the trick! Feel free to reach out with any questions or if there is anything else I can do to help!