I have an iPhone 4. It has network lock from Switzerland. I want use Gevey SIM on this phone. Gevey SIM is more compatible for iOS 5.1.1.
how to downgrade from iOS 6.1.3 to iOS 5.1.1 in iPhone 4?
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It's not possible. Each time you restore an iOS device's OS, Apple must sign as valid the version you are trying to install, and once a version has been considered as deprecated, which usually is a couple of weeks after a new iOS version has been released, Apple stops providing a valid certification for it.