You can change the units that your measurements appear in on fieldmargin between metric (m, m², km and ha etc) and imperial (feet, inches, acres etc).
Open the hamburger menu in the top left and select 'General Settings'.
On the settings screen you will find options to change your measurement units.
Click on the cog in the top right of the screen and select 'My Settings' from the menu.
A new window will open, click on the Units tab to select how your length, area and temperature measurements appear.