As of September 2015, Apple supports iOS 8, transitioning to iOS 9. If a device cannot run software newer than iOS 6, then it is possible to restore the device using an iTunes or iCloud backup or recovery mode, as instructed in Apple's support material. If a device already runs a newer version of iOS, installing iOS 6 in 2015 may require jailbreaking the device, which is legal according to the Electronic Frontier Foundation.
Apple supplies an electronic signature in iOS software, explains Macworld. When iTunes detects a connected device, it confirms the device's identity and the version of iOS currently installed, then checks Apple's servers to see if updates exist for the device. Because iTunes checks for the identifying signature, when Apple stops signing a piece of software, that software is no longer available in iTunes. Apple stopped signing iOS 6 in 2013.
Old versions of iOS do exist online, but updated signature information on a device stops even manual installation of old operating software, notes Macworld. Downgrading a device requires having this identity information saved from an earlier device state, explains the Mac Observer. iTunes backups contain this information, but overwriting or deleting a backup means this information no longer exists. Jailbreaking software allows users to restore old data files so the device sees the downgraded software as valid.