iOS 11.2.1 (build 15C153) is an official Apple over-the-air update for the iPhone 6s, released on 2017-12-13T18:13:33Z. The update package is 55.04 MB. This is a delta (partial) update that requires 11.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 731be1515f93305009f22c8ba37c0f16613c12c6.zip.