Consent should be managed self-serve by contacts on forms and on the consent and subscription preferences page.
However, manually updating consent status for contacts is possible, for example when importing contact lists or correcting an error.
Restrictions on bulk consent updates
When you're updating consent statuses in bulk there are two restrictions in place to ensure that you can't (accidentally):
Downgrade express consent status to implied consent
Resubscribe contacts who had previously unsubscribed
Updating consent in bulk
Follow the steps from this article.
Updating consent for a single contact
Find the contact then click on it to view the contact details page. From the contact details page click the "Change consent status" link.