To manage your site's menu you need to be using the menu component. This component is usually added to the default theme template, so you need to open that template for editing.
Start from the website manager and click the "Pages" link
From the edit pages page, find the "defaultThemeTemplate" template in the right hand side and click the edit button for it.
Note that you may have more orless templates in your list depending on what apps you've installed.
Once you're editing the theme template, hover over the menu component and click the cog to edit its settings
Scroll down to the bottom of the menu settings to see the list of menu items.
Click the green Plus button beside a menu item to add a child menu item inside it. The item called "Root menu item" contains all menu items which are initially displayed.
When items are added they initially have no content, you need to edit them to have the text and link you require
Click the pencil icon beside a menu item to open its settings
Some menu items are provided by apps and cannot be removed, but they can be hidden. To hide a menu item, change its visibility to "Hidden for all users".
You can also select a specific user group to be able to see a menu item, or you can make it only visibible to non-logged in users.
This is important, but easy to miss!
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