First, copy your gp2.exe to gp2.old.  To run the gp2chnum utility, go
to the GP2 main directory on your hard drive and delete gp1state.bak
and gp1state.sav. These are the game state files.  Then, run the
gp2chnum.exe file and follow all the prompts.  Then, run gp2.  Since
you had to delete the game state files, go into Options under the Main
Menu, then under Advanced Options, in the Options menu, and select
"Save Game State".  Note that by removing the game state files all your
saved settings have been replaced by the defaults, therefore you will
have to once again set ALL the options the way you like them, such as
joystick calibration, car retirements, session lengths -- the whole
ball of wax.

The author of the program recommended restoring your original gp2.exe
file after using the utility.  This probably isn't a bad idea.  One
note:  When I used this utility, it worked great until I restored a
saved game that used the old numbers.  This caused the game to reset to
the default old numbers. Therefore, it's a pretty good idea to delete
all your old saved games before you run this utility.