watchOS 5.1 (build 16R5584a) is an official Apple over-the-air update for the Apple Watch Series 4 (40mm, LTE), released on 2018-10-15T17:10:27Z. The update package is 120.59 MB. This is a delta (partial) update that requires 5.1 already installed. This update is no longer signed by Apple, meaning it can no longer be applied on-device through Settings.

Payload file: ab8e5ef6be04d04699eae692f0758ca58caf28d2.zip.