Save SHSH Blobs So You Can Restore Later
Those are the only two official ways to downgrade to previous versions of iOS. You can either downgrade from a beta version to a stable version, or downgrade to the previous stable version during a short window where the old IPSW files are still signed by Apple.
But, if a version of iOS is signed by Apple, you can use a tool to capture that “authorization” to run the previous version of iOS — in the form of “SHSH blob” files. These are device-specific digital signature files that control which versions of iOS can run on a device. Once you’ve received that authorization for your device, you can keep those files and restore that old version of iOS at any point.
For example, if you’re using an older version of iOS with a
jailbreak available but you want to play with the current version, you’d want to get local copies of those SHSH blobs before upgrading to the current version. This way, you can then downgrade to the older, unsigned version and continue using your jailbroken software.
You can use
TinyUmbrella — and possibly other tools — to create local copies of these SHSH blobs. Afterwards, you can use it to force iTunes to restore older IPSW files.