To manage your categories go to 'Configuration->Manage Vehicle Categories' depending on whether you have added any categories or not will depend on how this page looks. For now we will assume you have created some categories and your page looks like the one below:
To add a category go to the form at the bottom and fill in the name of the new category in the field to the right of Category Name and click the add category button to the right.
You will notice next to each category there is a 
icon. This is to delete a category but only if there have not been any vehicles added to the category yet. If there are vehicles added already you will have to edit the category instead.
To edit a category just click the category name and the name will change to a edit field like below:
Edit the text inline and hit return or click somewhere else on the screen to save the changes.