watchOS 4.3.1 (build 15T5555c) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2018-05-01T17:06:57Z. The update package is 361.44 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: c75853b3d89983174e157e4929a0ac549fbfa9a6.zip.