iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6, released on 2018-04-24T17:10:30Z. The update package is 679.06 MB. This is a delta (partial) update that requires 11.2.6 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 9aa62cf17e35c79fcf7423e8e715bf308bec7ec0.zip.