Usually we have the FTP port set as 21 in most of the servers, if we need to change the FTP port from 21 to another, the changes should be done in /etc/pure-ftpd.conf file.
Entry in which the changes should be done is;
# IP address/port to listen to (default=all IP and port 21).
# Bind 127.0.0.1,21
♦ Bind *,221
Here in this example we used the new port as 221 and * can be changed by the IP address if needed.