To manage your menus go to your Dashboard and go to Appearance and then Menus
You will a screen like this:
Along the top you'll see tabs for each of your menus. You can Identify which menu is your main navigation menu by seeing which one is selected in Navbar 1, or if you have a sidebar menu, which menu is selected in the Custom Menu widget in your sidebar. (See How do I edit my sidebar?).
Under each tab you'll see the pages or links that are currently part of your menu. Here you can simply click and drag to rearrange your menu items. You can click the little down arrow to open up a panel where you can change the menu label. You might want to use this if the page title is too large for the menu, for example.
To add a new item to the menu from your site, just tick the boxes next to the pages you want to add in the Pages box and click the 'Add to Menu' button.
To add a link to another website/webpage use the Custom Links panel. Here you can enter the web address for the page you want to add and type a label for the menu. Then simply click the 'Add to Menu' button to add the link to your menu.
To remove a menu item, click the little down arrow on the page or link you want to delete to open the panel and then click the bright red 'Remove' link.
To add a dropdown menu, just drag a menu item underneath and to the right of the menu item the dropdown will appear under. When you let go the item should appear indented. This indicates a 2nd level menu. You can indent pages under these second level menu items to create third level menus and so on. The videos available in your Manual will show you how to do this.
Video tutorials on how to edit and manage menus are available in the included help videos.