How to update iOS using cellular data network?

My uncle's using iPhone 3G and he wants to update his iOS to version 6 from version 5.1 but the problem is, the update is asking for a Wi-Fi connection whereas he just updated his iPad 3 to iOS 6 using cellular connection. Anyone have any idea

Added (1). It's an iPhone 3GS, my bad. So he's getting the option to update but only with Wi-Fi.

If you are using net on desktop or laptop connect iphone with it then install itunes in your laptop or in desktop then there you will see update in your iphone tab in itunes then wait until it fully download and it automatically update it.