iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6s, released on 2018-04-24T17:10:48Z. The update package is 729.76 MB. This is a delta (partial) update that requires 11.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: e312d7051e163da235af957293ac2859cc9254fb.zip.