watchOS 5.3.1 (build 16U600) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2019-08-26T17:15:55Z. The update package is 201.83 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: 6e48cdbe3ea31c2d50eb7bf975a0451740b48322.zip.