watchOS 5.3.1 (build 16U6600) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2019-08-26T17:16:10Z. 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: cfea2b44b1612bd02d0f34ab4e84c2b2e4ab6373.zip.