To switch between miles and kilometers go to your "User Settings" from the top right hand menu. Then scroll down to the "Unit Preference" section.
This setting is per user so will apply to only the logged in user. If a coach and athlete both want to use the same measurements then both the user accounts will need to be configured to match.
A coach can change their own settings but can also change an athletes settings by going to the "People - Members" option from the left hand menu and then click the users name and edit the unit preference from there.