watchOS 4.3.1 (build 15T5555c) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm), released on 2018-05-01T17:07:20Z. The update package is 80.36 MB. This is a delta (partial) update that requires 4.3.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: b6f8806da3e3876ef3667526e1d9ccfa85b36ef6.zip.