iOS 11.3 (build 15E5189f) is an official Apple over-the-air update for the iPhone 6, released on 2018-02-20T18:06:26Z. The update package is 297.79 MB. This is a delta (partial) update that requires 11.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 548e42cc8214d4d709fac78139c4e23c91d8faa4.zip.