Hey!
I've downloaded all the downloadable games this afternoon, feel free to update your game but to keep it fair upload a second file and call it "post jam" or something similar. That way people can see what was made during the jam, and after. Since you're the full owner over your own game (including the assets) you can also just add a new Itch.io entry and create a new page.
For HTML5 games it's a bit more difficult since you can't host two separate games on a single page, for that I'd say it's totally okay to do fixes but not to add new features. If you like to extend one of those games, make a second page as described above.