On the Speedometer segment click the Speedo Calculator button...
then on the Speedo Calculator tab:
enter your new tire diameter or tire size, click Apply;
then on the Shift Point Correction tab:
in the New Calibration Value pane, click the <- (left arrow) button,
and verify that your new tire diameter shows up;
at the bottom, click Select All, click Apply, click Close;
go File->Save As and save with a new filename.