If you just wanted to change the name of these items, those are editable via admin --> content --> String Resource Manager via the admin site. (eg. search for "menu.").
If you wanted to add or change the content of the menu itself, you would modify the menudata.xml file in the skins directory. The content for categories, manufacturers, etc. is generated dynamically, however you can insert your own items into the menu as well... Eg. if I add
Code:
<item Text="My New Item" NavigateUrl="somepage.aspx" LookId="DefaultItemLook"/>
Inside the customer service node, the customer service dropdown will contain a new item called My New Item, that links to somepage.aspx.