watchOS 5.1.3 (build 16S5531a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-12-19T21:06:18Z. The update package is 780.5 MB. This is a delta (partial) update that requires 4.3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 2c46320b37cfc8c09bdb8bb18df40272a137eb62.zip.