iOS 11.3 (build 15E216) is an official Apple over-the-air update for the iPhone 5s (GSM), released on 2018-03-29T17:11:43Z. The update package is 1.27 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: 0180e83204e34a05b209bc9f3605c510463e1c6e.zip.