Ghost offers two types of navigation: Main Navigation and Secondary Navigation. To add links to either of these two types of navigation follow these steps:
In the Ghost Admin go to Settings and then click on Navigation.
Either in the main navigation or in the secondary navigation, click on the last empty field.
Enter a label for the menu item you are going to add.
Enter the destination link for the menu item in the URL field.
Save the changes.
You can also change the order of the menu items by dragging and dropping each one in the desired position.
Note that the secondary navigation items are displayed within the "More" submenu if the site is being viewed in a desktop browser as you can see in the following screenshot: