I don't know. Never owned a Yaris. Read the manual. If you don't have that. Go to the Toyota dealer in your city. They will show you for free. Or find a U-tube video about it.
The reason why the fuel average is not moving is because the computer is averaging the fuel consumption ever since the meter was last reset. That may be thousands of kilometers ago so what you are seeing is the fuel average over all those kilometers. When you reset the fuel average and begin to drive, You will see the fuel average as you drive from the moment you reset it. It will change as your car covers distance. On my Civic, the reset button is on the instrument pod. On my Chery Tiggo, it is via a button on the steering wheel. On my Proton it is at the end of the indicator stalk. Don't know about your Yaris.
