watchOS 4.3.1 (build 15T5545b) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2018-04-16T17:04:24Z. The update package is 146.54 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: eb26c2c9d061966a47bc0a64534a645bf3b4768b.zip.