Earlier this week, Apple released a day one update for the Apple Watch Ultra which was delivered to customers on Friday. watchOS 9.0.1 addresses a bug with the speaker which could have distorted audio during phone calls using the built in speakers. This update is only for the Apple Watch Ultra and won’t be coming to other models.
To update go to the Apple Watch app on the iPhone, then to General > Software Update. Like all new updates for the Apple Watch, it needs to be on charge, within Bluetooth range of the connected iPhone and have at least 50% battery.