I don’t think you can do it through the bbPress interface at all ( I could be wrong about that, but I’ve never seen it. ) You would have to modify it directly in the database. Using a tool like phpMyAdmin find the topic in the bb_topics table, and modify the topic_slug. Make sure the slug you select does not already exist – the slugs need to be unique.
I thought maybe giving the thread a new topic would result in an updated slug, but it does not (slug does not change). I don’t see an option to change it anywhere, other than just modifying the bb_topic table directly.