iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6s, released on 2018-04-24T17:10:39Z. The update package is 1.34 GB. This is a delta (partial) update that requires 10.0.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 89923d8eabcb443f11c1114ff22438be2bc8e505.zip.