watchOS 5.3.9 (build 16U693) is an official Apple over-the-air update for the Apple Watch Series 3 (38mm, LTE), released on 2021-01-26T18:26:18Z. The update package is 499.61 MB. This is a delta (partial) update that requires 5.1.2 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: af0964a2ea1ee6fbeb2897a9922e6bf6ef5d8ced.zip.