iOS 11.3 (build 15E216) is an official Apple over-the-air update for the iPhone 6s, released on 2018-03-29T17:12:39Z. The update package is 1.31 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: 205d260d5f3d3c34d56c83607c998f983c6c0a5d.zip.