mileage display on the speedo can revert back to its originally programmed Kilometers option.
There really is nothing you can do yourself unless you own a MB Star kit.
Your best bet is to either go to a smart dealer or find a smart specialist in your area who can help.
It's a fairly quick and easy job for them, they just plug into the car, inspect the speedo
and select Miles instead of the Kilometers on their laptop and then log out. 5 minutes work.