In order to change the unit settings from Kilometers to Miles or visa versa please to the following:
From the application:
Open up the main menu in the lower right. Go to Settings - Account Settings - Units and choose between Miles or KM's.
From the Website:
Go to https://www.endomondo.com/settings and change the selected units.
If you're having issues with changing the unit settings for your Endomondo account, please contact our support team directly here.