You can't do that from the admin.

1) Get a good text editor like Notepad++ or Crimson Editor to edit php files.

2) Get an FTP client like FileZilla.

3) Download and unzip Zen Cart on your computer. You'll see the folder/file structure mentioned in all the tutorials/FAQ.

4) Follow the override system (look in the tutorials/FAQ), edit the file and upload the folder to your site.

It's a bit more complex than that, but once you start, you'll be better able to follow the info in the tutorials/FAQ.