Thanks for posting! Once a contact is unsubscribed they will need to resubscribe to your list on their own. You can encourage the contact to do this but the action must come from them in order to remain compliant with Can Spam and CASL laws.
The easiest way to have the contact resubscribe is to send them a Resubscribe email. Once you have selected the unsubscribed contact to resubscribe, they will receive a Confirmed Opt-in email where she will need to click a link in order to confirm her interest. As soon as the contact clicks the link in the email, she'll be added back to the lists you've selected. Below is what the email sent to your contact will look like: