iOS 11.3 (build 15E216) is an official Apple over-the-air update for the iPhone 6s, released on 2018-03-29T17:12:41Z. The update package is 774.48 MB. This is a delta (partial) update that requires 11.0.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 08abf76eebb717e2cd7c24a9c42b1f1493d69b74.zip.