If you wish to associate an email with another contact, you will need to unlink email from the contact it is associated with, then re-assign it to another contact. The 'Unlink' link (no pun intended) is located on the contact’s email message view pane:
If the message was archived, it will automatically become unarchived after you unlink it from contact.
NB: If you do not future emails from the same sender to be automatically assigned to the same contact, don't forget to remove that email address from the contact details.