Navigate to WordPress Dashboard > Appearance > Menus
From the drop-downs on the left, choose the content to add to your menu.
Adding Pages, Categories, etc. to your menu
On the left side of this screen are a number of drop-down boxes titled "Pages", "Links" and "Categories". Each heading will list available items you can add to your menu.
For example, to add a Page to your menu, please ensure you've first created the desired page (within WP Dashboard > Pages > Add New). Click the "Pages" drop-down heading. The pages you've created will be listed below. Check the box next to the page you'd like to add to your menu and click "Add to menu". The selected item should now appear under "Menu Structure" column. Here, you can drag and drop the item you've added to adjust its location in the menu. You can also create sub-menus by dragging and dropping a menu item underneath another so that it's indented.
Be sure to check the box to set the menu as "Primary menu"
Click the "Save Menu" button.