watchOS 5.1.3 (build 16S5531a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-12-19T21:06:15Z. The update package is 56.08 MB. This is a delta (partial) update that requires 5.1.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 3b47b770a683ab1658c2c1611f3faa41ef93a253.zip.