To change Android Auto temperature unit in widget, some posts suggested to talk to OK Google directly by saying "Change temperature to Celcius." However, that does not work all the times and most likely will perform a Google search on the "Celcius" topic instead.
To change the temperatue unit in Android Auto:
1) Start Android Auto,
2) Settting,
3) Google Assistant,
4) Perferences,
5) Weather,
6) Then set the desire temperature units.