Hi Robin,
Just press the mode button until it dosplays 'trip 1'
Then hold down the mode button (for at least than 10 secs) or until the display changes and shows length.
Now using the 'set' and 'mode' buttons, you should be able to toggle the length (diamerter) of the wheel back to 1870mm as the manual states.
Once you have the correct setting, press the 'mode' and 'set' buttons silmultaniously for at least 2 seconds to exit
Viola