Your site's menu is managed with the menu component. This component is in the default theme template, as it is typically applied across all pages. 

1 Go to Content, then Edit web pages, and open the defaultThemeTemplate. 

2 Click the settings cog on the menu component to open the settings.

3 Scroll down to menu items and click the plus button next to Root Menu Item to add a new menu item. Clicking plus next to an existing menu item will add a child menu item underneath it.

When items are added they initially have no content, so you need to add the required link and text.

4 Click the pencil icon next to a menu item to edit it.

5 Add the path and menu item name, and add an icon if you would like icons displayed in your menu.

6 Click Select Group Visibility to select who should be able to see the menu item, then save your 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.

7 Click Save menu - this is very important, but easy to miss!