After I updated the /etc/xinetd.d/telnet , what service / daemon need to re-start to make the setting vaild ? is it /etc/rc.d/init.d/xinetd start ?

Thx in advance.

If the file you changed is a file xinetd uses as a configuration file, then you need to restart xinetd. I haven't messed with xinetd directly, so I don't honestly know, but it looks like it. Anyway, you can either use your system startup scripts to restart xinetd, or do it manually.

If your startup scripts file Red Hat's convention, then you should be able to execute something like this on the commandline:
The name of the script may be different; you'll just have to explore a little.

To restart it manually, you can use the kill command:
The syntax may be different for your system. Do a man killall to see. The SIGHUP signal forces the process to restart itself, causing it to re-read configuration files.

Otherwise, you can restart the machine.

thx your reply ,

but I have one more queston , what is the difference between /etc/rc.d/init.d/xinetd and /etc/init.d/xinetd , is it the same ? thx

Look at the long listings for the directories:
ls -ld /etc/rc.d
ls -ld /etc/rc.d/init.d

One directory is probably a symbolic link to the other.
/etc/init.d -> /etc/rc.d/init.d
So the file entries refer to the same file.