All that I have read indicates I should be able to upgrade the IOS  of a Iphone 3gs from ver 4.2.1 to 4.3 or 5.0 at the least. Most apps have been upgraded to 4.3 minimum. All unavailable to me using ver 4.2.1. How can I upgrade to 4.3  or are there probs

Why cannot the Iphone 3gs be upgraded from ios 4.2.1 to 4.3 or 5.0

Perhaps yours is an iPhone 3G and not a 3Gs...
Connect to iTunes on the computer you usually Sync with and “ Check for Updates “...
If an Update Appears Install it... if not... you are up to date for your particular Device...
See the Using iTunes Section Here...
How to update your iPhone, iPad, or iPod touch

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