watchOS 5.3 (build 16U5526f) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2019-05-15T17:11:21Z. The update package is 131.56 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: aad49f0c829420c4ba888ccd236f2f462f80698f.zip.