watchOS 5.1.3 (build 16S535) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-01-22T18:11:13Z. The update package is 969.06 MB. This is a delta (partial) update that requires 3.1.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 21e964b08ebc95fee0054c7d06e66f56ccfd401d.zip.