Your storefront has two parts to the navigation. The header navigation and the footer navigation. Changes to these navigation menus will be reflected throughout your storefront. With some customization you can have different headers or footers for different pages.

Click the three dot menu ... and Edit and menu item. It's easy to add new menu items and even sub menus. Drag and drop items around to re-arrange them.

You can also add custom links:

Custom links will open a new tab as long as you append https:// to your link.