iOS 9.3.1 (build 13E238) is an official Apple over-the-air update for the iPhone 4s, released on 2016-03-31T16:54:33Z. The update package is 19.39 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: da844488a5029207bce742b66f23c74afb575640.zip.