iOS 9.3.1 (build 13E238) is an official Apple over-the-air update for the iPhone 5s (GSM), released on 2016-03-31T16:55:08Z. The update package is 25.97 MB. This is a delta (partial) update that requires 9.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 3e937bc4300bf31b53ac0f29fd9820f1524ef930.zip.