iOS 11.3 (build 15E216) is an official Apple over-the-air update for the iPhone 5s (GSM), released on 2018-03-29T17:11:42Z. The update package is 1.3 GB. This is a delta (partial) update that requires 10.2.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: f1a2c8138c85e9c95963d49412b222ee4382f25b.zip.