watchOS 5.3.1 (build 16U6600) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2019-08-26T17:15:59Z. The update package is 146.84 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: f162144b9130f6b2ece1294358b6142b5bab40ae.zip.