iOS 12.1.3 (build 16D39) is an official Apple over-the-air update for the iPhone 6s+, released on 2019-01-22T18:11:32Z. The update package is 1.4 GB. This is a delta (partial) update that requires 11.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 97f6890466fb6d8403f30a78780380506849367e.zip.