watchOS 5.2.1 (build 16U5091b) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-04-08T17:12:38Z. The update package is 144.05 MB. This is a delta (partial) update that requires 5.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 72a2d4237d9a7ebd42f1ba7858b377223790214e.zip.