Make a post saying 'bump'. If you need to bump again, and the last post is a bump too, delete it and re-post.
You have to wait some time before you are able to bump your own threads (assuming no one has posted yet) in certain sections (market, mainly). If someone posts during that waiting period, you'll be allowed to post too.
There is no 'bump' option on Sythe, and won't ever be implemented.