On your website admin, browse to Appearance > Customize > Menus > Select a Menu > Add Items. When you scroll to the bottom portion of the settings screen you will see a checkbox allowing you to “automatically add new-top-level pages to this menu”.
Just click on the checkbox next to “Automatically add new top-level pages to this menu”, this will automatically add all the new top-level pages in your website to this menu.
Some would prefer to not automatically add new pages to the menu as adding multiple new pages, more than the design capacity allows for, can break the menu appearance making your website look less professional. The purpose of menus is to serve logical and relevant pieces of information to customers through targeted menus. Besides, adding new pages or custom links to a menu is as simple as clicking a button so we would advise to keep this option disabled.