watchOS 5.1.3 (build 16S535) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-01-22T18:12:02Z. The update package is 781.67 MB. This is a delta (partial) update that requires 4.3.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: e8aff346156c60a70d09f825a905904274123ede.zip.