watchOS 4.3 (build 15T5199f) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-03-06T18:08:17Z. The update package is 336.64 MB. This is a delta (partial) update that requires 4.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 4390627a9f2e183a680c4ea338d44ff32ef0200e.zip.