watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-12-10T18:10:04Z. The update package is 734.83 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: e40a6c62a9b2f87bfbedbed9faa2f9b71ef00fd3.zip.