watchOS 5.1.3 (build 16S5531a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-12-19T21:06:13Z. The update package is 734.95 MB. This is a delta (partial) update that requires 4.3.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 8e9b55b9e89d14b615692a3c3ba488e571bf521e.zip.