If you are using the phone app, go to the "Account" section at the top left corner, then scroll down past everything to near the bottom for Settings. Select Advanced Settings then find the Units section and choose the units you'd like your data to use.
On the webpage dashboard go to;
https://www.fitbit.com/settings/profile
Scroll down to advanced settings and change things there.
It appears you have more flexibility if you use the phone app.
Remember to sync your device afterwards.
