watchOS 5.1.3 (build 16S535) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-01-22T18:11:16Z. The update package is 748.73 MB. This is a delta (partial) update that requires 4.2.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 61258c365ebda7d1d6e4c618d3388762543ccebc.zip.