watchOS 5.3 (build 16U5537b) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm), released on 2019-05-28T17:12:43Z. The update package is 150.35 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: f26c61d372233cc48a77a35cdacfa77fd46ac481.zip.