watchOS 4.3 (build 15T5199f) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-03-06T18:08:13Z. The update package is 324.41 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: 40bc7fbb5686438d0a4689a5d4a906fdfdd89895.zip.