You'll need the following tools:
1. A copy of the nav OS, current OS version is V32
2. NavTweak, google for it.
3. NavTweak requires the GDK runtime environment, google for it
4. An ISO image editor of your choice, eg MagicISO

Then (using NavTweak) replace the GIF image in the OS with a new GIF image, ensuring the GIF image size is approx the same.

Burn the image onto a CD

Then load the OS onto the nav.