On some occasions, when you disconnect the speedo wiring or let the car battery die, the total 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.