I believe you click on it in garage and left to the change car their should be a 99 symbol that allows you to change the number. Thats what where it is for other racecars but karts im not sure.
Change Racing Modification number: Select your RM car in your garage -- look for the option immediately to the left of 'Change Car' and 'Tuning'. Four different fonts, and numbers 1 through 99.
