The file is owned by root. Open up a Konsole. You will find an icon in the start menu. At the $ type in SU and press enter. You will have to type in your root password. You should have set it at install time. If you are sucessful, the prompt will change to a # sign.
As root you have power to do just about anything, so be careful. If you must operate with a gui, launch konqueror, type konqueror at the command prompt. Konqueror can be used to show directories graphically. There is a navigation bar on the left with some icons. If you don't see it, press F9, it should appear. Click on the red folder. This should show your root directory. It looks windoze like. From hear navigate to the file of choice. Click it, it may be double click on your system. That can be configured for convienience. Single click will save your left mouse button and make it last years longer....
That should open it up, with system default editor. Linux has many editors, so have a look at what is installed, and find one you like. Edit the file, and save.
When you close the editor and Konqueror will appear. To close the console, type exit twice. Hope this gets you going.