iOS 11.3 (build 15E216) is an official Apple over-the-air update for the iPhone 6s, released on 2018-03-29T17:12:44Z. The update package is 679.35 MB. This is a delta (partial) update that requires 11.2.5 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 91353d1fe5963b2c2d6893f6a5ff4c890e83d6d7.zip.