watchOS 5.3.1 (build 16U6600) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-08-26T17:16:11Z. The update package is 127.2 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: 4a85698a95250a2b197a7f73fc864230e750be41.zip.