Apple recently released iOS 15.1.1, and as is traditional, Apple has now stopped signing iOS 15.1, making downgrading to that version of iOS no longer possible.
Apple in mid-November released iOS 15.1.1, which fixes some bugs that were affecting iPhone 13 users. According to the company, previous versions of iOS 15 have a bug that causes phone calls to drop unexpectedly.
The new software can be downloaded on compatible devices over-the-air by going to “Settings” -> “General” -> “Software Update.”
Falling back to an older iOS build is often used by users that jailbreak their iPhones. Restoring an iPhone or iPad to a previous version of iOS can also sometimes be helpful for users who experience significant bugs after upgrading to the latest version of iOS.