That's not updating the content of a torrent, that's updating a torrent file given a URL and a signature. It's not even BitTorrent, more like embedded RSS. Doesn't really help us out here.
The nice thing about BitTorrent is that if you point the new .torrent file at the same location as the old one, your client should incrementally download just the new parts. All you need is a way to distribute and switch over to new .torrent files automatically - something like BEP 0039, for example.
