By renumbering. I’m guessing you have duplicate shells and want to change the numbers out or just one number. First you need to find appropriate decals. Pretty much exact if your just removing one or two numbers.
Removing the numbers without damaging the paint is the hard part. Wet sanding with very fine paper will work. My go to method for hoppers or freight cars. Is a Paasche air eraser. Just mask off exactly what you want to remove. Work slowly and it basically erases the number or numbers. Best to practice on a scrap car first. Hit the area with some clear gloss as the erasing will take the shine off the paint and place your decal. Then clear again
You may may need numberboards also. There’s a few methods for doing these.
If you can describe your project as to what you want to change up and what color or road name your working with. It will be easier to help you it.