Open a terminal and type:
and there it is!
You can also "find" it by opening up your file manager and going to "file system" and then the "etc" folder, then the "apt" folder, then the "sources.list" file will be there -- it is just a text file.
To edit it you will need to open an editor as root using `sudo`
sudo nano /etc/apt/sources.list
Use <Ctrl>+x to exit, then press "y" and Return to save the modifications.