There are a couple of ways to do this, but probably the simplest is to use the GUIs:
1. Get two copies of the file manager window up,
2. a. move one to the directory where the new copy of sources.list is (if it's not already there),
b. move the other to /etc/apt/ (go up to the root directory to find etc.)
3. you won't be able to do anything in b, as a user so click on Tools at the top of the window and select "open current folder as root". Once you've given the root password you can manipulate files in the folder - but be careful you can do harm here!
4. Then find sources.list in this folder and rename it e.g. sources.list.old (just in case).
5. After that you should be able to drag the new sources.list file into this b window from window a.
6. That's it!
If you are familiar with Linux/UNIX commands you can do this from the command line in a terminal (as root).
I've just seen DeBaas's method I was assuming you'd already got the sources.list in a text file, if not follow what he says.
