SET @tid = 39475, @ts = UNIX_TIMESTAMP(STR_TO_DATE('Saturday, August 24 2013', '%W, %M %e %Y'));

UPDATE phpbb3_topics t
  LEFT JOIN phpbb3_posts p
    ON (t.topic_id = p.topic_id)
  LEFT JOIN phpbb3_forums f
    ON (t.forum_id = f.forum_id)
  SET
    p.post_time = @ts,
    t.topic_time = @ts,
    t.topic_last_post_time = @ts,
    f.forum_last_post_time = @ts
  WHERE t.topic_id = @tid;