iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6s, released on 2018-04-24T17:10:40Z. The update package is 1.34 GB. This is a delta (partial) update that requires 10.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 544d0f7ce8d610c7b83af323796cbd2837ad4d70.zip.