Apple appears to be in the process of decoupling the iPhone from the watchOS update process on the Apple Watch. 

In the first beta of watchOS 6, the Apple Watch gained its own over-the-air software update mechanism under Settings > General > Software Update. As on the iPhone, the mechanism checks for any available watchOS update and, if prompted, downloads the update directly to the Apple Watch. 

The mechanism became functional on Monday when Apple seeded the second beta of watchOS 6 to developers. 

As noted by Jeremy Horwitz, however, users still must review and accept the watchOS terms and conditions using an iPhone. Hopefully this is a temporary step that Apple removes before watchOS 6 is publicly released in September. 

Updating an Apple Watch's software previously required using the Apple Watch app on an iPhone for the entire process. 

This change is in line with the Apple Watch gradually becoming more independent from the iPhone. In 2017, for example, Apple Watch Series 3 models launched with cellular capabilities for making phone calls and more without an iPhone. And in watchOS 6, the Apple Watch received its own on-wrist App Store.

Source: MacRumors