As you can see from the red dot on the top of the watch face, you have pending notifications. Swipe down from the top to look at the notifications.
Now that you can see all the notifications in a vertical list and wish to clear all of them together, force press the screen. Force press is a very hard press on the screen as if you are trying to push the glass on the watch inside.
On force pressing the display, a new button will popup saying Clear All, as shown above. Tap the 'X' once and all the notifications would get cleared at one go.