iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6s+, released on 2018-04-24T17:11:03Z. The update package is 683.99 MB. This is a delta (partial) update that requires 11.2.5 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 68f21088ab359891a932047f8d15844d392a7f30.zip.