watchOS 5.3 (build 16U5526f) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm), released on 2019-05-15T17:11:56Z. The update package is 147.46 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: 4eb7114903b108524720a2fd82eea167ca882af2.zip.