watchOS 5.1.3 (build 16S5523a) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm), released on 2018-12-10T18:10:35Z. The update package is 195.1 MB. This is a delta (partial) update that requires 5.0.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: f50779937697d1cb8e25fe89d8d67251903085fb.zip.