iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 6s+, released on 2018-04-24T17:10:59Z. The update package is 740.89 MB. This is a delta (partial) update that requires 11.1.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: b3c85656bb8e5423682828dfad5f2bd6130cdd98.zip.