iOS 12.1.3 (build 16D39) is an official Apple over-the-air update for the iPhone 5s (GSM), released on 2019-01-22T18:09:37Z. The update package is 398.85 MB. This is a delta (partial) update that requires 12.0 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 3460fe35d820ad0acdb38ba19e27bc940cbc403e.zip.