iOS 11.2.1 (build 15C153) is an official Apple over-the-air update for the iPhone 6s, released on 2017-12-13T18:13:32Z. The update package is 310.63 MB. This is a delta (partial) update that requires 11.1.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 4352820a706bbad386e9e1bb703622f579791c19.zip.