iOS 9.3.1 (build 13E238) is an official Apple over-the-air update for the iPhone 6s, released on 2016-03-31T16:55:23Z. The update package is 345.96 MB. This is a delta (partial) update that requires 9.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: bd4569daf1e95697591c65c5e23431a768d92afa.zip.