iOS 11.3 (build 15E216) is an official Apple over-the-air update for the iPhone 6s+, released on 2018-03-29T17:13:01Z. The update package is 690.43 MB. This is a delta (partial) update that requires 11.2.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 609c131137d7e9553d6bea6eaf0c10cbbd8f15b9.zip.