watchOS 5.3 (build 16U5545c) is an official Apple over-the-air update for the Apple Watch Series 1 (42mm), released on 2019-06-11T17:13:34Z. The update package is 127.89 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: 698daee5cab24292c598ee7a8961bec1ece914fd.zip.