watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2019-08-26T17:15:56Z. The update package is 142.29 MB. This is a delta (partial) update that requires 5.2.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 8eadff9c9b7adeda279ee231b7ca4abd248ba303.zip.