watchOS 5.1.3 (build 16S535) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-01-22T18:11:17Z. The update package is 748.73 MB. This is a delta (partial) update that requires 4.2.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: d1799e8ba1df5afef24e7c4af6201dd811dbc5b5.zip.