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All About Downgrading
Downgrading your iPhone firmware is a way to return to a previous version of the iOS. Many users find that they want to downgrade their iOS after accidentally upgrading and losing their jailbreak or unlock. Other users find that upgrading to some of the newer iOS versions such as 4.0 has slowed their devices drastically and therefore wish to downgrade.
In the past the iPhone firmware files were not checked against the Apple server when users wanted to downgrade. This essentially meant that anybody could downgrade without any special tools needed. Unfortunately Apple has changed that in recent times and iTunes checks the "signature" of the iOS files before allowing you to install them. Apple does not allow users to downgrade to previous versions of the iOS, therefore by default iTunes will not allow you to downgrade your iPhone firmware.
There is one method to downgrade your iPhone firmware. Before attempting to learn how to downgrade iPhone firmware it should be noted that you will need to have your ECID SHSH Certificate for the previous version. Jailbroken users can back this up to Saurik's server inside of Cydia. You will not be able to downgrade your iPhone if you do not have your ECID SHSH Certificate. Therefore if your iPhone was never jailbroken, you have no hope of downgrading at this time. There has been no progress in downgrading without the ECID SHSH Certificate and likely will not be.
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How To Downgrade
Assuming that you are fortunate enough to have backed up your iPhone ECID SHSH Certificate the downgrading process is actually quite easy.
- Open the hosts file. Open the following file location on Windows, "c:\windows\system32\drivers\etc\". Mac users should navigate to "/etc/". Locate the file called "hosts" and open it in a text editor.
- Change the hosts file. At the end of the hosts file you need to add the following text, "126.96.36.199 gs.apple.com". This will route iTunes to Saurik's server when searching for the ECID SHSH Certificate rather than the Apple server.
- Place iPhone in recovery mode. Unplug your iPhone from the computer and power it down. Press and hold the home button on the iPhone and reconnect the USB cable to the computer. After a minute or so you should see the "Connect to iTunes" screen on the iPhone, you can now release the home button.
- Select firmware. Open iTunes if it is not already open. Click the device in the device list on the left side of iTunes. Hold down the shift key (Windows) or the alt key (Mac) and click the restore button. Select the firmware file you wish to restore, this should be the firmware version that was on your iPhone prior to the last upgrade.
- Wait. iTunes will now complete the installation of the firmware. Once the installation is complete you will have successfully learned how to downgrade your iPhone firmware.
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Tips & Resources
You may come across tutorials on the internet that tell you how to downgrade the iPhone firmware without the ECID SHSH Certificate. These are outdated tutorials and do not actually work with the newer versions of iTunes and the newer iPhone firmware. Essentially you will need to have the ECID SHSH Certificate if you plan to downgrade your iPhone.
You can download older iPhone firmware versions here. Make sure to choose the appropriate version for the appropriate model of iPhone as they are not all made equally.