iOS 11.3 (build 15E216) is an official Apple over-the-air update for the iPhone 6s+, released on 2018-03-29T17:12:57Z. The update package is 1.42 GB. This is a delta (partial) update that requires 10.3.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 42e6e436c698bb0b97cd0f552d080a0d9d819625.zip.