watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-12-10T18:10:07Z. The update package is 137.93 MB. This is a delta (partial) update that requires 5.1.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 3b106e1100cf688caad3774bea1b1fda50aa1c3f.zip.