iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 5s (GSM), released on 2018-04-24T17:09:54Z. The update package is 717.83 MB. This is a delta (partial) update that requires 11.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 34b1addbda8dabae004964f13a5e4f493dead7b1.zip.