iOS 12.1.3 (build 16D39) is an official Apple over-the-air update for the iPhone 6+, released on 2019-01-22T18:10:13Z. The update package is 1.35 GB. This is a delta (partial) update that requires 11.3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 38ad18fae4c8d8a6a096e9e3c6e0fffb0ae272c4.zip.