watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-12-10T18:10:17Z. The update package is 122.3 MB. This is a delta (partial) update that requires 5.1.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: f122e66e7b75381e2c981c20a8e27a362b0f132a.zip.