watchOS 5.2.1 (build 16U5091b) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-04-08T17:12:32Z. The update package is 458.11 MB. This is a delta (partial) update that requires 5.1.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 7eba0776172e2ab248a395e9a5d47bebdb88e541.zip.