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I have updated iOS version to 4.3.5 but I want to reinstall 4.2.1. Does any one know how to reinstall 4.2.1 on iPhone?

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migrated from Aug 11 '11 at 16:50

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Please add more details: your device, generation, whether it has been jailbroken. – Usavich Aug 11 '11 at 15:38
This can be achieved by modifying your computers host file to tell iTunes to check on a server other than apples for signing of this downgrade. Apple will NOT sign for downgrades, because they don't want to make jail breaking any easier for you. If you could provide me with more details I could help you through this process, for example do you already have the EXACT build of iOS you want to downgrade to available, and are you running on a mac or a pc? – Mick MacCallum Aug 11 '11 at 16:57
iTunes will never on it's own allow you to go backwards. You will need to be very specific about the exact model of iPhone and need to have saved some files to go backwards. Do edit the question to let us know more details on which phone model you have. There might be other software that will work if you have saved some files used to sign the OS 4.2.1 and make iTunes think it's currently valid for a restore. – bmike Aug 11 '11 at 19:13
up vote 2 down vote accepted

This might be not possible, depending on what device you have. Because Apple has stopped signing 4.2.1, unless you have backed up your SHSH, there is no way back on iPhone 3GS or 4.

If you have jailbroken before, Cydia might have backed up your SHSH. Then you should use Cydia's server to perform a restore.

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no idont have any backup for tht n never use jailbroken – Pooja Aug 11 '11 at 15:37
@Pooja what type of device are you trying to downgrade? – Usavich Aug 11 '11 at 15:40
i want to downgrade iPod touch – Pooja Aug 11 '11 at 15:43
If you have a 3rd or 4th generation of iTouch, you are out of luck. – Usavich Aug 11 '11 at 15:54
that was what I was getting at – Alex Coplan Aug 11 '11 at 16:57

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