watchOS 5.1.3 (build 16S5535a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-01-07T18:16:21Z. The update package is 175.5 MB. This is a delta (partial) update that requires 5.0.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 5169d337d3ac7204c95d8bfac8862d2131774d59.zip.