iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone 5s (GSM), released on 2018-04-24T17:09:55Z. The update package is 666.22 MB. This is a delta (partial) update that requires 11.2.6 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 7a339f99b887d5dd9418f73c0ee50180420418cb.zip.