watchOS 5.2.1 (build 16U5113a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-05-07T17:18:40Z. The update package is 448.86 MB. This is a delta (partial) update that requires 5.1.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 182e18305298c0696acbf786a724a72b52563d17.zip.