How can I can update from my current version 4.2.1
How I can upgrade my current version
iPhone 3G, iOS 4.2
How I can upgrade my current version
iPhone 3G, iOS 4.2
If you have an iPhone 3G, as your profile says, you can't. iOS 4.2.1 is the end of the line for your phone. There are no further updates available.
If you have an iPhone 3G as stated in your signature, you can not upgraded beyond iOS 4.2.1.
How can I can update from my current version 4.2.1