How to add an existing or newly created page under Menus in Wordpress
Step 1: Go to Appearance and click Menu.
Step 2: Kindly refer the Menu structure in the screenshot below, we are going to add an already existing page "Introduction to Linux" under the menu "Linux".
Step 3: Perform the following steps to add "introduction to Linux" under Menu.
Step 4: You can find the newly added page "Introduction to Linux" at the bottom menu structure.
Step 5: Drag the "introduction to Linux" and place it under "Linux".
Step 6: Click on down arrow button near page
Step 7: Choose under Linux option
Step 8: Introduction to linux is successfully added under Linux.
Step 9: Hit Save menu button to save the changes made to the website.
Screen shot shows the diffrence in actual webpage before and after the change using wordpress