iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6s+, released on 2018-04-24T17:10:56Z. The update package is 1.46 GB. This is a delta (partial) update that requires 10.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 2b01a1ab7da0e1c5516d89ea66de0c155612ab34.zip.