In the backend of the website, select "Design" > "Menus" from the menu on the left. Here you can change the order of the menu items via drag & drop.
New menu items can be added on the left. Menu items that link to external pages are added via the "Individual Links" tab. New menu items are always added at the end of the menu.
The text that is displayed in the menu can then be changed at the respective menu item. (The menu items are expanded by a click and the settings appear). The "navigation label" is the displayed text in the menu, but does not change the link to the page itself.
If you want to delete a menu item, you must click "Remove" at the very bottom of the settings that open.