Update a contact's name, fields, or tags, or unsubscribe it

Change name, merge fields (a null value removes a field), change tags (tags replaces the whole set; add_tags and remove_tags change it), or set status to unsubscribed. A contact can never be resubscribed through the API: only the recipient can, by confirming a signup form, which clears their earlier unsubscribe from that list and from the workspace but never a bounce or complaint suppression.