iOS 9.2.1 (build 13D15) is an official Apple over-the-air update for the iPhone 4s, released on 2016-01-19T17:59:40Z. The update package is 897.79 MB. This is a delta (partial) update that requires 8.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: c0c197036fc4955b01c6478703ee8750505df0b8.zip.