Tap 3Dots (Menu)
Select Language & Units
Then select options that you want. If you select Automatic it should change to Miles in UK and Kilometres on the Continent.
PS. In case you don't know it is recommended at the moment that you turn the unit off rather than putting in Sleep mode as they have managed to identify a Bug with the Lane Guidance system which is causing some users problems. They are trying to resolve but in the meantime that is a recommendation.