watchOS 4.3 (build 15T5209a) is an official Apple over-the-air update for the Apple Watch (42mm), released on 2018-03-12T20:07:02Z. The update package is 306.82 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: 91badbdf77390ba06b3d744f8c2cbb4898713464.zip.