Can I update my old iphone 3g from ios 4.2.1 to ios 5.0?

I can't install some apps because apps store says I need ios 5.0. If I can how do I it.

You can't the 3G is stuck at 4.2.1

You can't, because of minimum hardware requirements don't matches with it… I wonder with iphone 3gs too.

Can't. Apple does you to download the last compatible version of the apps though.

IPhone 3GS can be updated to iOS 6.1.3