[@robkk Moderator 11 years, 4 months ago]

add this to your child theme functions.php or install the plugin below.

function bbp_reverse_reply_order( $query = array() ) {
    $query['order']='DESC';
    return $query;
}
add_fiter('bbp_has_replies_query','bbp_reverse_reply_order');

https://wordpress.org/plugins/bbpress-sort-topic-replies/