Set unit preferences in the app and on the website. (Note: units are stored locally on the device; setting units on the web will NOT change the unit settings in the mobile app).
Changing Units in the Mobile App:
Tap the Menu button (top left). Then go to Setup & Help and choose your preferred units.
(Note: Images are from Android app (ios and updated versions may vary slightly.)
Changing Units on the Website:
Select the My Weather drop-down menu and go to Settings.
Select your preferred units and be sure to save your settings by choosing Done.