watchOS 6.3 (build 17U216) is an official Apple over-the-air update for the Apple Watch Series 5 (44mm, LTE), released on 2021-01-26T18:27:31Z. The update package is 133.55 MB. This is a delta (partial) update that requires 6.2.5 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: 748fd91151832c30402b2805a1b186390947a533.zip.