Dock accessibility

According to the "Related Article:"

Use the Dock on your Apple Watch

Quickly open your favorite apps or go from one app to another. 

Open an app from the Dock

1. Press the side button.
2. Swipe up or down. Or turn the Digital Crown. 
3. Tap to open an app. If you scroll all the way down, you can tap All Apps to go to the Home screen. 
4. To close the Dock, press the side button.

You would press the side button

See https://support.apple.com/en-us/HT206992