I want to change quit message of mIRC Options by command and Current version of mIRC does not.

It does not? Just write "/quit quit-message-here".

This is not changing any mIRC option, just quits from the connected server.

There are a lot of options for which there is no command to change them. In this case, a simple workaround is to put something like "%quittext" in the options field. Then you can use "/set %quittext My quit text goes here" to change the default quit text at any time.

You can alias the /quit command in an alias script to perform your own quit msg.