I followed this tutorial How to Change Your FTP Port :: The cPanel Admin and other similar tutorial, to change ftp port, but still cant make it

the problem is, /etc/chkserv.d/ftpd file always back using the original file, before i edit the file, that still using port 21,

after i restarted the ftp server, using these commands
/etc/init.d/pure-ftpd restart
/etc/init.d/cpanel restart (restarts tailwatchd/chkservd)
or from whm > Restart Services > FTP Server (ProFTPd/PureFTPd)



please help guys why that file always back automatically using the original file, or give me the working tutorial link to change ftp port




Nb.
- i am using Pure-FTP
- i already disabled configserver firewall, so there is no problem with firewall