to the left of the steering wheel you should have the multi function lever. you should have 2 controls on it: the rotary control and the SET/RESET button.
scroll the menu, using the rotary control, until the board display shows "YOUR SETTINGS >> SET/RESET". then press the SET/RESET button once. the car has to be stoped (motionless) when you do this, otherwise this menu will not be available (it is a safety feature).
after pressing the SET/RESET button, the menu will change and you will have to scroll (using the rotary control) to the option named "MEASURE UNITS METRIC". press the SET/RESET buton again, set the desired option, and then scroll to "YOUR SETTINGS >> EXIT".
as Richard told you, this will change many things, not only the temperature.