Apple is no longer signing iOS 17.5.1 and iPadOS 17.5.1, which means you can no longer downgrade from iOS 17.6 and iPadOS 17.6.
iOS 17.5.1 and iPadOS 17.5.1 no Longer Signed by Apple, Downgrade from iOS 17.6 and iPadOS 17.6 not Possible
As long as Apple is verifying older firmware, you can downgrade to that software version on your iPhone and iPad. When it comes to iOS 17.5.1 and iPadOS 17.5.1, Apple had been signing the firmware for quite a while, but it recently just stopped signing it. This means if you have upgraded to iOS 17.6 or iPadOS 17.6, you can no longer downgrade.
There are a bunch of reasons why people choose to stick with older firmware on iPhone and iPad, mainly because it serves them better in some regard, such as battery life. I usually recommend upgrading to new iOS and iPadOS as soon as it comes out because it features extremely important security fixes most of the time, and in some cases, crucial bug fixes.
If you are currently on the fence about whether you should upgrade to iOS 17.6 and iPadOS 17.6 or not, don’t be. Just go to Settings > General > Software Update and install it right away. It will protect your device against malicious attacks, adds stability when it comes to day-to-day usage and offer better compatibility against newly released software and hardware.
