watchOS 4.3 (build 15T5187e) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-02-21T18:11:58Z. The update package is 334.98 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: e207f4735721bf15d5eb19dfa5d637536a1b974f.zip.