watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-12-10T18:10:06Z. The update package is 136.61 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: 236f8b80b8d166b726822a7f047f6d721bcd963e.zip.