You can see the contacts you have Unsubscribed under the 'Unsubscribed Contacts' tab within the 'Relationships' page.
To remove an unsubscribe tag on a contact (to re-subscribe them), you can click into the contact and hit the 'Unsubscribe' button.
You will then get a pop-up message that will give you the option to 'Remove Unsubscribe'
This contact will now appear within your 'Everyone' tab on the 'Relationships' page!