iOS 9.1 (build 13B143) is an official Apple over-the-air update for the iPhone 4s, released on 2015-10-21T17:16:00Z. The update package is 174.35 MB. This is a delta (partial) update that requires 9.0.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 58bb0f75d9eac58e8eb2a708b57fc1c623a4497e.zip.