How can I repair wrong /etc/rc.conf ?

Tue Mar 9 05:45:23 UTC 2010

I'm using FreeBSD 6.4 + windowmaker in IBM notebook.
I edited /etc/rc.conf to use LAN other ip address is different from my home
address in my school.
I got error message from this
[
/etc/rc.conf: 131: Syntax error: Unterminated quoted string
Enter full pathname of shell or RETURN for /bin/sh:
]
so that I returned, I decided to edit /etc/rc.conf using vi.
But freebsd didn't find vi, less any other application.
I can't modify even if read /etc/rc.conf.
fortunately I can use cat so I found my fault sting from /etc/rc.conf but I
can't modify any files cause the system changed into read only file system.
How can I repair /etc/rc.conf file with fault statements using any editor?

Thanks in advanced.
Oscar