To modify your navigation menu, you need to visit the Appearance » Menus page in your WordPress admin dashboard.
First, click the View All tab to see all your site’s pages
Check the box next to each of the pages you want to add to your menu
Click on the Add to Menu button
Once your pages have been added, you can move them around by dragging and dropping them
Note: All menus have their items listed in a vertical (top to bottom) list in the menu editor. When you put the menu live on your site, it will either display vertically or horizontally (left to right).
After adding pages to the menu, select the location where you want to display the menu and click on the Save Menu button.