You should never do any modifications to the Growtheme or Growbox .php files directly. Beside the risk that something goes wrong and the theme or plugin becomes unusable, all of your modifications would be gone with the next update of the Growtheme or Growbox Functions Plugin.
Thats why its common best practice to do any modifications you might want to do in a childtheme.
To make this progress easier for you, you can download a sample childtheme by clicking here. Of course you will still need to have both the Growtheme and Growbox Functions Plugin installed to make the Childtheme work.
Once you have downloaded the files to your computer, you can simply upload it like a normal theme to your wordpress install.
After you have installed and activated the theme, you can connect to your WordPress Installation via FTP and make any modifications within the functions.php file.