Your COROS device and the COROS app do not share the same unit settings. You have to configure the setting on the device and the app separately.
On COROS devices
From the main menu, scroll to System > More Settings > Units. Select Units to toggle between imperial (feet) and metric (kilometers). Select Temperature to toggle between Fahrenheit and Celsius.
On the COROS app
From the Profile page (fourth tab on the bottom), select Settings and scroll down to Units. Here you can toggle between imperial (feet) and metric (kilometers). You can also select Temperature to toggle between Fahrenheit and Celsius.
Please note, it is not possible to customize different units for different metrics (ex: imperial for distance and metric for elevation).