watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm), released on 2018-12-10T18:10:25Z. The update package is 150.89 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: bc98c92a2552fff8fe9f456136c5ea6b77a1452b.zip.