watchOS 5.1.3 (build 16S5535a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-01-07T18:16:26Z. The update package is 31.89 MB. This is a delta (partial) update that requires 5.1.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: a62d342ca26140e1e2ed95c412481ce5bc64f8ee.zip.