If you'd like to add an unsubscribe link for your leads so they can easily opt out of your mailing lists, you should go in Campaign - Sequence section and you'll find the unsubscribe button in the sequence editor here:
You can customize the text overlay for your unsubscribe link:
This is how your email looks within the sequence editor and a received email:
FAQ
Q: What do my leads see when they click on the unsubscribe button?
A: When your leads click on the unsubscribe link, they will see this: