Add Menu Items to an Existing WordPress Menu
Add Menu Item(s)
In the vertical menu on the left, hover over Appearance and click on Menu. Click the box next to the page that you want to add to the menu and click the Add to Menu button. The new menu item will show up at the bottom of your menu list. Drag and drop the menu item to where you want it in the menu structure.
Click on Save Menu to complete your menu change.