watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-08-26T17:16:03Z. The update package is 877.06 MB. This is a delta (partial) update that requires 4.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 452be203506e89366bec54bb373b63bd046d52a6.zip.