To edit your navigation bars, go to Design -> Navigation. You will see that the page is divided into links in the top navigation bar, and links in the bottom (foot) navigation bar.

To add a link, just click the appropriate 'Add' button, fill out the name, and select what you want to link to. (
Pages, Product Category or Webaddress

To delete a link, hover your mouse over the link and then click the "Delete" button that appears.

You can also drag and drop the links to rearrange the order within the navigation bar. Note that if you want to move a link from the top navigation bar to the bottom one, you will need to delete it from the top and then add it to the bottom. The same goes for moving from bottom to top.