watchOS 5.3 (build 16U5537b) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-05-28T17:12:15Z. The update package is 450.67 MB. This is a delta (partial) update that requires 5.1.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: dcc0258dfd91ee1f4d1c8425bd0eb357257e19f6.zip.