watchOS 4.3 (build 15T5199f) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-03-06T18:08:16Z. The update package is 663.37 MB. This is a delta (partial) update that requires 3.2.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 32b9c7bba303d61accc5cb2d2951a5711628080d.zip.