iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6s, released on 2018-04-24T17:10:46Z. The update package is 774.27 MB. This is a delta (partial) update that requires 11.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 683096b7d260c0bd06f3c3ebf112a292b902178d.zip.