watchOS 4.3.1 (build 15T5545b) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-04-16T17:04:19Z. The update package is 62.65 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: a4d3f28355f87667dfd5f17071c0efe2e545f4a9.zip.