iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6, released on 2018-04-24T17:10:23Z. The update package is 774.08 MB. This is a delta (partial) update that requires 11.0.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 87fc2badd57ee2bf76f2bf9019cd2dea06b2b96f.zip.