So ...

There are two different ways to edit your functions.php file ...

1. You can use the default Wordpress editor ...

Simply select the functions.php file to load its content, insert the new code and click the "Update File" button.

2. You can use an FTP connection or a file Manager plugin to download/edit/upload the functions.php file.

Personally, I prefer the second method because it will allow you to create an offline backup copy of your original functions.php file.

Now let's see the actual code and the result ...