Cant change ftp port, chkserv.d/ftpd file always back using original file

I followed this tutorial thecpaneladmin.com/change-ftp-port/ 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