watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-12-10T18:10:18Z. The update package is 222.78 MB. This is a delta (partial) update that requires 5.0.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 199c76a3a82d39129d771d09e94b7a78eaf3a5a5.zip.