iOS 11.3 (build 15E216) is an official Apple over-the-air update for the iPhone 6, released on 2018-03-29T17:12:31Z. The update package is 679.22 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: 43af28617b5500ba235e9663bc74bf1b51709dcf.zip.