watchOS 5.1 (build 16R5584a) is an official Apple over-the-air update for the Apple Watch Series 1 (38mm), released on 2018-10-15T17:09:36Z. The update package is 90.8 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: 336569d4c7010c8c03c5ae537beb954ae59b7bc0.zip.