iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 5s (GSM), released on 2018-04-24T17:09:53Z. The update package is 1.3 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: 1fb1feff94fb2a9a9a885416cc3b46e7fbf20c78.zip.