What kind of upgrade? If you want a software upgrade go to settings, then general, then software update. If you want a hardware upgrade, go to http://www.apple.com/iphone/ . If you want app upgrades, go to the App Store and tap updates.
Hope this helps!
There is no such thing as an iPhone 2G. There was the original iPhone, then the 3G, 3Gs, 4, 4s and now the 5.
If you have an original iPhone (1st generation) then the highest version of iOS it can run is 3.1.3. If you have an iPhone 3G, the highest iOS it can run is 4.2.1.
Neither of those first and second iPhone models can be upgraded beyond those versions of iOS.
You would have to buy a new phone if you want to run any iOS version higher than those. The iPhone 3Gs can run the most current, iOS 6.1.3 but it will not be able to run iOS 7 when that is released this autumn.