watchOS 4.3 (build 15T5209a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-03-12T20:07:31Z. The update package is 324.31 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: 126fb0c37c4fd4c5ceaf960c2b5105af48d8d2e3.zip.