You can add a new page by clicking Add Page at the top right hand of the dashboard. This will take you to a channel entry editing page. Once you save the entry, go back to the CMS dashboard (the Website tabe) and you will see the page you just added now as a parent nav item. You can also add a child page (i.e. sub page) by rolling over the parent page. You will notice Add Child Page appears.
To edit a page simply click the page's name.
You can easily re-order pages by moving the mouse over to the left of the title (the cursor will change to a hand icon) and then clicking and dragging the order of the page. You can optionally drag sub pages outside or within other pages so that they become either parent or child navigation items.
Delete a page by clicking the Delete button which appears when you move your mouse over a page.