JsonAPI – Updating the relationship for a resource

I am having trouble understanding http://jsonapi.org/. Say, I have the following endpoints in my API for authors and articles.

/authors

/authors/:id/articles

If I wanted to fetch the details for a single article, I can use

/authors/:id/articles/:id

which returns data that looks like

{
  "id": "1",
  "type": "articles",
  "data": {
    "attributes": {
      "title": "Programming in javascript.",
      "body" : "Some long text here..."
    },
    "relationships": {
      "author": {
        "data": {
          "id": "2",
          "type": "authors"
        },
        "links": {
          "self": "http://api.com/authors/2"
        }
      }
    }
  }
}

Now, if I want to update this article and change the author, the spec says I should use a PATCH. Should this PATCH request be made to

/authors/:id/articles/:id

/authors/:id/articles/:id/relationships/author

or

/articles/:id

/articles/:id/relationships/author