iOS 9.3.1 (build 13E238) is an official Apple over-the-air update for the iPhone 4s, released on 2016-03-31T16:54:30Z. The update package is 932.09 MB. This is a delta (partial) update that requires 8.0.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 856ad3ce57a8d9d9ba5649dc0c7fc2014ef5c66f.zip.