There are two methods of resubscribing a contact:
Method #1 - Web Form
Add the contact via a Web Form that you can create with the help of Elastic Email (learn more here).
Method #2 - Subscription link
Go to your Elastic Email account's Settings "Unsubscribe" tab. Here you will find a "Subscription link" that you can copy and forward to your contact outside of Elastic Email. Once clicked, the user will be forwarded to a default subscription form.
With both methods, after the contact is added, they will be sent an email with an activation link - when the link is clicked, the contact's status will change back to "Active".