Hi Jorg,
Please use this plugins. This plugins may help to implement the google analytics on your site. Or if you want to add the plugins without using plugins then you may go to the theme folder (interface pro) -> inc -> functions -> functions.php and enqueue your google analytics script as we have already done on line no 78 to 84.
But each and every time you update to our theme the same thing you need to add while updating to our new version. Because while updating to our new version all the customisation code will be lost. So we always recommended you to create a child theme before making any changes on it.
Thank you!