watchOS 5.1.3 (build 16S5531a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-12-19T21:06:15Z. The update package is 1.25 GB. This is a full OTA update that can be applied from any compatible base version. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 054c0805575e57adda99e5ee58665383b279fafb.zip.