How do I update my IOS with mobile data?

You don't, because you can't. You need Wifi or you can use you computer with iTunes to do the update. If you don't have a computer or Wifi, go somewhere where you can access Wifi.

If you have a computer and you want to update using your computer see this link and follow the directions for Update using your computer --> Update your iPhone, iPad, or iPod touch - Apple Support

You can’t