This question already has an answer here:
I bit the bullet and upgraded to iOS 9.1, and the phone now moves like it's an iPhone 3GS. I can see iOS 8.x is no longer signed, but fortunately I have SHSH of iOS 8.2 saved.
How can I downgrade without losing my warranty? I've heard jailbraking will void warranty, but I guess jailbraking then flashing an iOS version that removes the jailbreak should allow my phone to still be in warranty.
PS: This is not a duplicate of Can I install any iOS version I wish by picking and choosing which firmware to restore?, because it talks about restoring from IOS9 to usinged IOS8 version with SHSH blobs.