watchOS 4.3.1 (build 15T5555c) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-05-01T17:07:06Z. The update package is 361.47 MB. This is a delta (partial) update that requires 4.2.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 196ab5aefb52367e4f25d3ba62492fbbb77f7567.zip.