watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-12-10T18:10:16Z. The update package is 182.22 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: b39272b065face9f55d2fe0dc5dc87f8b6454697.zip.