The root directory of an FTP account is the topmost folder in the directory hierarchy. You will be able to see, from the selected folder, its contents and the contents of its child directories.
To modify the root directory of an FTP account, you must access the dashboard of your Hosting Service. Then, you must access Service Management → FTP Management:
You will need to locate your FTP account. Then click on the menu [...] → modify FTP account.
In this new window and from the Select directory section, mark the new root directory for your FTP account.
Note that the FTP account will not be able to access any directory above the root directory.
If the FTP account is assigned or linked to WordPress, you will not be able to modify its root directory.
To create an FTP account see the manual How to create an FTP account