You can assign a tag to contact on the Contact Details page. To open this page click on the contact email or View button on the All Contacts page. Then scroll to the Tags section on the right of the Lists section.
To add a tag to a contact click Add Tag. The following dialog will open.
You can select a tag from the drop-down list or enter a new tag. You also can delete already added tags by clicking on the cross on the right of each tag. After you finish editing tags click Save.