watchOS 5.3 (build 16U5558b) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-06-24T17:25:00Z. The update package is 81.44 MB. This is a delta (partial) update that requires 5.3 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 93f39d7ae90d91f50ea729742d3c51ebbceeea29.zip.