watchOS 5.3.1 (build 16U6600) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-08-26T17:15:19Z. The update package is 64.04 MB. This is a delta (partial) update that requires 5.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 7563eb953013dddfec6cb748bbeab74f8fcb2c5d.zip.