iOS 11.3.1 (build 15E302) is an official Apple over-the-air update for the iPhone SE, released on 2018-04-24T17:11:16Z. The update package is 672.77 MB. This is a delta (partial) update that requires 11.2.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 687ce4a7a50242ee290d9cb005ae6b47169a61ca.zip.