watchOS 4.3 (build 15T5209a) is an official Apple over-the-air update for the Apple Watch (42mm), released on 2018-03-12T20:07:04Z. The update package is 306.5 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: 809c27577ef9ef0f0391680bbbb340150b523b61.zip.