I have an 05. On the back of the speedo, at about 5:00, there is a button, push once for "A", push again for "B". I use "A" for gas mileage and "B" for trips....
The guy I bought my 883 from used masking tape on the front of the speedometer to track his miles. I read the manual and found the button on the back...much more user-friendly than tape!