If you know the ID of the author you can use wp_insert_post specifying the ID and the author ID to it.

    $id = $post->ID; // change this to whathever
    $user_id = '4'; // change this too

    $the_post = array();
    $the_post['ID'] = $id;
    $the_post['post_author'] = $user_id;
 
    wp_insert_post( $the_post );


The trick is to specify the ID to update the post. See [`wp_insert_post()`](http://codex.wordpress.org/Function_Reference/wp_insert_post).