This is in your theme files….BUT I would definitely recommend you access it using an FTP program (you can do a google search for a tutorial on how to setup and use one). You’ll also need an html/css/php editor to edit the files.
You can use the WordPress editor if you want, but beware, if you make an error it will take your entire site down and show a white screen displaying the error, and I haven’t seen a way to undo it since theres no way to go back……Thats why it’s recommended to use FTP and an editor (nearly all developers/designers do this) as you can easily undo any mistakes. But if you’re going to use the WP editor, just make sure you copy and paste everything perfectly to avoid mistakes.