Hi,
To change the password for the system/shell user admin, get a root shell and run the following:
CODE
passwd admin
then set the new password for admin when prompted to.
As your goal is to login by ftp, I highly suggest that you not use the same password for the shell user admin as for the plesk user admin. Doing so will mean that you are sending your plesk administrator user/pass through plain text whenever you login by ftp. If you do need to ftp in as the system user admin, you should instead use sftp.