I think that you have a router, which is connected to the internet? If so, you have to apply a portforwarding from Port 80 (and 443) to IP 192.168.64.3 in your router.
Please keep in mind, that Xampp is NOT designed for Online servers but for local development only.
So you want to make your website accessible from your local area network (LAN). XAMPP can already do that by default, you don't need to change anything.