After logging into WordPress (www.yourdomain.com/wp-admin).
  1. Click on Appearance > Menus.
  2. In "Select a menu to edit" dropdown, choose the menu that you want to add a link to.
  3. Click Select, to load the menu.
  4. In the Pages window on the right, click on: Links.
  5. Enter the URL of the page you want added to the menu
  6. Enter the Link Text - This is the name you want displayed in the menu for your new link.
  7. Click the Add to Menu button. Note: The page will be added to the bottom of the menu in the menu tree.
  8. Find the page in the menu tree.
  9. Click and drag the page to the appropriate position in the menu. Note: If it is a Child page of a Parent page, you will want to position it so that it is indented below the Parent page.
  10. Click the blue Save Menu button.

See image below for numbered steps:
User-added image