iOS 9.2.1 (build 13D15) is an official Apple over-the-air update for the iPhone 4s, released on 2016-01-19T17:59:41Z. The update package is 819.13 MB. This is a delta (partial) update that requires 8.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: b3bb5a9bf323d42e4afce65f3ace8dd38f05a852.zip.