How do I change the number on a number plate in gimp?

You cannot

Are you talking number plates the white and yellow ractangular things on the front and back of the cars or are you talking about the boxes that get put on skins that are designed for your number. As both are know as number plates. If you explain what you mean can be answered alot quicker
The cars number plates, afaik, cannot be altered. As for the number plates on the skin, i don't know either as i dont use gimp.

All I shall say is that you can change it only for the CMX viewer.