watchOS 4.3 (build 15T5212a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-03-16T20:09:27Z. The update package is 324.47 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: 36e3aeb7fefe56e0facea442d39d93df7c8466ef.zip.