watchOS 5.3.9 (build 16U693) is an official Apple over-the-air update for the Apple Watch Series 4 (44mm, LTE), released on 2021-01-26T18:27:07Z. The update package is 494.62 MB. This is a delta (partial) update that requires 5.1 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: 4a91afb7361289fd6848ea998b7eefdce5cc7521.zip.