iPhone 4 ios6 update

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I just bought a used iPhone 4 iOS 5.1.1. And I did some research on different telephone providers.but I had few questions on some of the providers and updating my iPhone. Like I said the phone is now running on iOS 5.1.1. I had it on h20 wireless running on 3G but only had 2gigs of data. If I reached that data I had no more Internet until next month. I need much more than that, I listen download and browse the Internet daily. So I changed to net10 wireless also running on 3G ,so they say. They said up to 5gigs in data then it slows to I believe is edge Internet . But since I got it I'm not able to stream any videos at all, so I don't believe is 3 g or is limited to videos. I can listen to online radio but no video. Q: Can I upgrade to ios6 without unblocking the phone and will it work just by inserting the sim card? Because they told me it runs on AT&T towers. Answers anyone Thanks : miami206