By default, vsftpd (Very Secure FTP Daemon) utilizes port 21. To change this port is a fairly quick process.
$ sudo nano /etc/vsftpd.conf
In this file, find
listen_port. If you can't find it, add it in yourself.
Then restart vsftpd.
$ sudo service vsftpd restart
Make sure you allow this port to be used through your firewall. If you're using CSF, add the port to the TCP_IN and TCP_OUT connections.
$ sudo nano /etc/csf/csf.conf
# Allow incoming TCP ports TCP_IN = "20,21,25,53,80,110,143,443,465,587,993,995,202" # Allow outgoing TCP ports TCP_OUT = "20,21,25,53,80,110,113,443,202"
$ sudo csf -r