watchOS 5.1.3 (build 16S5531a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-12-19T21:06:16Z. The update package is 735.73 MB. This is a delta (partial) update that requires 4.3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: ca98aca146cc38cb90f19a76ec58267b421a10dc.zip.