Your ftp username/password is same as your control panel (cpanel) username/password. You need to login to your cpanel typically located at http://yourdomain.com/cpanel or http://yourdomain.com:2082/
Once you login, you can find the link to Change Password. That changes the password for ftp/cpanel/ssh.
If somehow you have lost the username/password of your cpanel, you can open a support ticket and ask for username of your particular account and get the password reset for it.