watchOS 4.3 (build 15T5209a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-03-12T20:07:22Z. The update package is 325.07 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: ba898c687f7ebbf6d5b151c2d9a4f20e83d815fb.zip.