Watch OS updates are handled through the Apple Watch app on your iPhone.

While Apple Watch updates are similar to iPhone updates, because the Apple Watch is paired to the iPhone for internet access and fast device management, Watch OS updates aren’t done directly on the Apple Watch, but rather on the Apple Watch for iPhone app. To get started, make sure you have your iPhone handy, and your Apple Watch charged to 50% or more.

How to update your Apple Watch using the Apple Watch for iPhone app

  1. Launch the Apple Watch app on your iPhone.
  2. Tap on the My Watch tab.
  3. Tap on General.
  4. Tap on Software Update.
  5. Tap on Download and Install.
  6. Enter your iPhone Passcode.
  7. Tap Agree to the Terms and Conditions.

The Watch OS update will download and transfer to your Apple Watch, your Apple Watch will reboot to apply the update, and then you’ll be all set.
from iMore – The #1 site for iPhone, iPad, Mac, and all things Apple!