how to edit the /etc/samba/smb.conf file

I cannot see the Workgroup anymore and am unsure when this became a problem.
The consequence is that I cannot print using the printers that are attached to a networked Win XP computer.
Forum help says that I should edit the SAMBA smb.conf file, but when I attempt this, I am blocked by a dialogue box that says Access Denied.
I am the only user on this laptop.
The system is Ubuntu 10.4.
Hardware is Asus laptop F6S. 32 bit 4Gb RAM
The work group was established long ago when all the networked computers were Windows. Now most are Ubuntu.
In the smb.cong file the workgroup name is correct, except that it is in lowercase instead of Uppercase.
I was, in the past, able to print from this laptop under the current setup to the networked printers.
So, in summary, there are two issues:
1. How can I fix the setup to be able to print? and
2. Why can't I edit the smb.config file anyway?
Greg