On the watch:
Access settings by pressing the top button to unlock the screen and swipe down on the home screen showing the main watch face. On the quick control screen, press on the purple gear icon to access the main settings menu. Scroll down to the "Common" section and press "Units" to switch between Metric and Imperial units.
In the app:
Open the "Amazfit Watch" app, click on "Profile" then "Settings" then "Units" and adjust to your preference. (km/mi, kg/lb, cm/ft).