watchOS 5.1.3 (build 16S5535a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm), released on 2019-01-07T18:16:36Z. The update package is 149.52 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: 270c63e91aef2b709cbc0cb724317a2b1e8e2bf7.zip.