watchOS 5.3 (build 16U5537b) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-05-28T17:12:19Z. The update package is 104.22 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: c9b3340a0f67a0114005435ada62e20cf42bfff2.zip.