watchOS 5.1.3 (build 16S5531a) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-12-19T21:06:23Z. The update package is 182.16 MB. This is a delta (partial) update that requires 5.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: c793724138840ed86cbbe1a4ed160106e52b8058.zip.