watchOS 4.3 (build 15T6212) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-03-29T17:09:05Z. The update package is 80.99 MB. This is a delta (partial) update that requires 4.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: def1f2ad5b1e9cd1cc51bf09df19bcda9fa66915.zip.