iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6s, released on 2018-04-24T17:10:49Z. The update package is 685.13 MB. This is a delta (partial) update that requires 11.2.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 216932e2aca7b7ec882efe26e46a09f2e11f54c2.zip.