The Navigation buttons are all images. You can change these images if needed.
First find out the name of the current button image by accessing your website.
Right click the image and select properties. For example the link will looks as follows:
Here about_us_90x35.gif is the name of the image for the button named About us.
Now go to 'Shop Design' -> 'Manage template images' and rename about_us_90x35 to
something else. This is just to make sure you have a backup of the image if you
want to use it later.
Now upload your image by following the FAQ: Managing images of the eShop?
Rename the file to about_us_90x35.gif
Now the image will be replaced.
The changes will appear online when the shop is re-published.