I don't think you can change it from days to miles. The FSS normally displays a certain number of miles left until service, but if it has been a long enough period of time and that many miles have not been reached, it will display days to service. It's like how most places say change your oil every XX months or YY miles. The FSS displays which ever one comes first. Assuming you bought your car new (leftover), it has been sitting for over a year with the same oil in it. That is why it is displaying days.