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I am using iphone 3GS. My phone os version is 4.1. I want to upgrade to 4.3 but the latest version is 6.0. How can I upgrade to 4.3 or 4.5 from 4.1?

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4  
You can't. Apple stopped signing that particular version some years ago. Not possible. –  cksum Jan 25 '13 at 6:49
2  
Why do you want an older version of the OS than the new current version? –  Mark Jan 25 '13 at 11:42
    
Because of speed decrease? –  Max Ried Aug 9 '13 at 19:09

2 Answers 2

Download the firmware iOS 4.3.5 from Apple:

http://appldnld.apple.com/iPhone4/041-1965.20110721.gxUB5/iPhone2,1_4.3.5_8L1_Restore.ipsw

and then perform the update with this file. You select to install from a local file by holding the Option key while pressing "restore iPhone" button in iTunes.

You can find a list of all available firmware files at various locations – use your favorite search engine to search for "iphone firmware files" and you'll find e.g. these lists:

http://iszene.com/thread-29469.html

http://www.iclarified.com/entry/index.php?enid=750

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3  
This doesn't get you the needed crypto keys to install it. –  Max Ried Aug 9 '13 at 19:10

Jump over to this website http://www.felixbruns.de/iPod/firmware/ which has every iOS for every model of phone. Simply find the correct model for your phone and click on the iOS version you want and download.

Once downloaded, follow the instructions on the top of the site (copypasta below)

How to restore or update your iPod/iPhone/iPad:

After downloading the desired firmware below, you can shift-click (PC) or option-click (Mac) the Restore or Update buttons in iTunes. A file dialog will open and let you choose the downloaded ipsw-file.

Note: Your browser may change the ipsw-file into a zip-file. In that case, just rename it to end in .ipsw and iTunes will accept it.

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Don't do this. Your phone won't activate since apple no longer signs old iOS versions. –  Kevin Grabher Mar 24 at 1:10

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