watchOS 5.1.3 (build 16S535) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm), released on 2019-01-22T18:13:28Z. The update package is 188.14 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: 4e4418d36eddf5ae5821faa54ff147529d72086d.zip.