how to edit file since my WP admin is blank?

Sorry, but I am new to this. I wrongly copied a line of code at the end of a function.php file, and my whole website is blank. I use WP within the google cloud platform. I was able to use sftp (filezilla) to check that all files are still intact. But I would like to delete that line of code, and filezilla does not allow it because the file is write protected (and filezilla only uses a public key).
Any suggestion is appreciated. This must be quite easy for you, but again, I am new to this.