Both Celsius and Fahrenheit units are supported to show the current temperature on Google Now. To change weather units Google Now, do as follows:
1. Open Google Now (Google application) on your device)
2. Find the Weather card, and tap exclamation icon on the top right
3. Select "Weather units"
4. Choose either Celsius or Fahrenheit
** If you search Weather in the Google search bar, then you just need to tap C or F, to toggle between Celsius Fahrenheit