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

    $aidi = $post->ID; // change this to whathever
    $useraidi = '4'; // change this too

    $tehpost = array();
    $tehpost['ID'] = $aidi;
    $tehpost['post_author'] = $useraidi;
 
    wp_insert_post( $tehpost );


The trick is to specify the ID to update the post. See http://codex.wordpress.org/Function_Reference/wp_insert_post