Thanks for updating the thread.
As you don't have child theme installed, you need to install Child theme to make the necessary changes.
You can use following resources to download and setup child theme.
Download Child theme from following source: https://theme.co/apex/child-themes
Please take a look at following article to setup child theme:
Please note that Filezilla is a software. You can get in touch with the hosting company to get FTP details.
That being said, if you would like more simpler solution you also have the option to install 3rd party plugin to add verification code. You can use plugins like Insert Headers and Footers. Here's the resource that can be used to download and get started with the plugin.