watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-08-26T17:16:06Z. The update package is 218.71 MB. This is a delta (partial) update that requires 5.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 73c1118cce32f942c9f1b831c6951ca4adc7391c.zip.