watchOS 5.3 (build 16U5537b) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-05-28T17:12:18Z. The update package is 129.91 MB. This is a delta (partial) update that requires 5.2 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: d95f90ef0cac819586533c99c003d584918a2325.zip.