watchOS 5.1.3 (build 16S5531a) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm), released on 2018-12-19T21:06:35Z. The update package is 147.64 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: cacc475978b49895d0adea0e3ccca168206b8edf.zip.