watchOS 5.3 (build 16U5526f) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-05-15T17:11:33Z. The update package is 82.56 MB. This is a delta (partial) update that requires 5.2.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 16b5697c97ff2f592b2a9f6f9bd2989fb242fd4f.zip.