watchOS 5.1.3 (build 16S5531a) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-12-19T21:06:21Z. The update package is 100.66 MB. This is a delta (partial) update that requires 5.1.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 7f26b56de6d41351d48ba64879f93a380ed3c93e.zip.