what is the procedure to update iphone 3gs ios 4.1 into ios 6.0

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See Here...

http://support.apple.com/kb/HT4972

OR...

Connect to iTunes on the computer you usually Sync with and “ Check for Updates “...

See the Using iTunes Section Here...

How to update your iPhone, iPad, or iPod touch

Connect your phone, iTunes running, on the Summary Pane, hit the "Check for Updates" button. Backup first!