Anyone who has opted out of your newsletter can resubscribe at any time by using the link to your Sign Up Form to re-opt in to your newsletter. That link can be found under Forms > Web Forms/Emails.
To copy the link, scroll down and click Share the form and then Get Link.
Once you click Get Link, the link is automatically copied. You can then paste it into an email and send it from your own personal email address to theirs. They can then click the link, fill out the form, and resubscribe.