iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6s+, released on 2018-04-24T17:11:02Z. The update package is 690.32 MB. This is a delta (partial) update that requires 11.2.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 8241715ed2cb2517862b24f0c7499a594d96afe3.zip.