watchOS 5.3 (build 16U5537b) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-05-28T17:12:24Z. The update package is 104.21 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: 3a0cbae05682bc6d4994454fd1bdae22b8c887f4.zip.