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 894.64 MB. This is a delta (partial) update that requires 8.1.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 635cf745b56a6afc82092abd2e3c16f04a20959c.zip.