Here is how to change your FTP ports and also activate passive mode on Debian Linux with Plesk.

Log in to your ssh client (like putty) or your local linux machine.
Open /etc/proftpd.conf with your editor of choice.
Put PassivePorts num1 num2 into your tags (where num1 and num2 are numbers between 1024 and 65534. This port range will be taken for Passive connections.
Change Port to some number between 1024 and 65534.
Save your file and do a restart of your ftp app.
On plesk it would be done like this:
/etc/init.d/xinetd restart

And don’t forget to update your firewall rules!