Head to the WordPress Settings tab under Admin Panel > Settings > WordPress Settings.
Click Edit Homepage.
Click on the Header Menu you wish to customize, and a control panel will appear on the left, with the Menu Selection Dropdown. Click Menus Screen, highlighted in blue.
You'll be redirected to the Menus Screen page. Choose the menu you wish to customize.
You can then go ahead and change the menu accordingly. Menu items can be dragged left or right to establish the menu hierarchy.
You can also add new Custom Links from the Add Menu Items section on the left.
Remember to click the Save Menu button to ensure the changes are saved.
You can also refer to this video tutorial.