Hi Christoph, thank you for contacting us!
If you want to edit the information contained in the <head> element of your page then the best way to do so is to utilize a plugin like Insert Headers and Footers (https://wordpress.org/plugins/insert-headers-and-footers/). Alternatively you can modify the header.php file directly by using your server's File Manager system and locating the file in your theme's directory, but that technique has some drawbacks because of how theme updates work in WordPress.
If you insert your own code into the theme files then all of your changes will get removed when you update the theme in the future. The plugin method stores its modifications separately and will persist through theme updates.
I hope that helps you change the meta data as needed and please let us know if there is anything else that we can do to help!
Customer support service by UserEcho