Please try visiting your control panel login screen (ex: www.your-domain.com:8443) and click on the link for 'forgot your password' - it can be emailed to you.

Once you're able to login to the Plesk control panel you can change your FTP password there if you wish. ( > Home > Domain Administrator )

Otherwise, please open a support ticket with us here: http://my.dotcomhost.com/support/ - request to have your password reset and please include the last 4 digits of the credit card number we have on file for you to verify your identity.