watchOS 4.3.1 (build 15T5555c) is an official Apple over-the-air update for the Apple Watch Series 3 (42mm, LTE), released on 2018-05-01T17:07:05Z. The update package is 361.61 MB. This is a delta (partial) update that requires 4.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: e0c1c50440bdaa932522fd63fc95d0770b73dacf.zip.