This feature was added to the first beta of the watchOS 6. You could access it by going to Settings > General > Software Update. But now after the release of beta 2, it is functional.
But according to the tweet by Jeremy Horwitz, users still need to accept the Terms and Conditions on their iPhone in order to update their watch. But it seems to be an temporary step which Apple might remove in the future updates.
Sure seems like the stage is being set for direct-to-Apple Watch software updates in watchOS 6, though it hasn’t been fully freed quite yet from the need to approve things on the iPhone. pic.twitter.com/pf3wBC3vU5
— Jeremy Horwitz (@horwitz) June 17, 2019
This is a step towards making the Apple Watch independent of the iPhone. watchOS 6 already brings in its standalone App Store, and with a few more updates the Watch will be more usable without your iPhone.