Falls du auf einer mySQL Datenbank arbeitest musst du glaub ich das SELECT in extra Klammern setzen. Also so...
Code:
UPDATE "phpbb_topics" SET "phpbb_topics". "topic_first_post_id" = (SELECT MIN( "phpkit_forumpost". "forumpost_id" )
FROM "phpkit_forumpost"
WHERE "phpkit_forumpost". "forumthread_id" = "phpbb_topic". "thread_id")