1 - Open Webhosting Manager
First, access the Web Hosting Manager by clicking "Open Webhosting Manager." Instructions can be found here: Where do I manage my webspace?
2 - Primary User
Click on the primary FTP user's username in the top right corner and click "Passwort ändern" in the drop-down menu.
Enter a new, secure password on the next page.
3 - Additional Accounts
To change the password for additional accounts or create new ones, select "FTP-Konten" -> "Zusatzkonten" from the menu on the left.
Here you'll find an overview of the existing additional accounts. Click "Passwort" to set a new password for the corresponding account.