Go to **Systems Settings -> Locale -> Country/Region & Language**. Click the "Other" tab. You should see a setting for "Measurement system" which you can set to metric. If you are in a country that uses metric by default, you should be able to do this automatically by selecting the right one in the "Country" tab.
If that doesn't work, right click on the widget to see if it has settings of its own. If not, you are out of luck.