iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6+, released on 2018-04-24T17:10:17Z. The update package is 32.5 MB. This is a delta (partial) update that requires 11.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 42b046c4293355cab9b10b418b028364ed17e497.zip.