On your Dashboard, hover your mouse to Appearance and click on Menus when a dropdown menu shows up.

On the Add Menu Items, check the page you want to be added on the Menu.

And once you have checked the desired page you want to add on Menu, click on Add to Menu. Then it will be added on the Menu Structure.

On the Menu Structure, Drag the newly added Page to where you want it. You can reorganize the Menu by dragging it.

Once you are done, click on Save Menu to save the changes you made.