watchOS 5.1.3 (build 16S5531a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-12-19T21:06:17Z. The update package is 134.83 MB. This is a delta (partial) update that requires 5.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 465e000095b70a87adfc4cd3e2f82f77e232d925.zip.