iOS 11.3 (build 15E216) is an official Apple over-the-air update for the iPhone 6s, released on 2018-03-29T17:12:38Z. The update package is 1.34 GB. This is a delta (partial) update that requires 10.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 35b12afaf16f40406d86aa7dad9460a0a6060ab7.zip.