watchOS 4.3 (build 15T5209a) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-03-12T20:07:29Z. The update package is 336.52 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: 271c86cea9d62c9e6edca7af60c742262aa10ebd.zip.