watchOS 4.3 (build 15T5187e) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-02-21T18:11:49Z. The update package is 335.47 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: 112e2ecf0927db4a21eee0a41e8759014b014f2b.zip.