Paris, France tourist visa 15 days. What can I do to extend it?

It depends on your visa type. If the visa is marked not to be extended, that is, it cannot be extended, then there is no way. You must leave Schengen before your visa expires. If you can extend your visa, you can apply to the French immigration office for visa extension immediately after you arrive in Paris. Italy and France are both Schengen countries. It doesn't matter if you fly directly. If you can't handle it, you must come back, otherwise the application will be very troublesome in the future!