Hi,
As I understand, in your WordPress admin menu you don’t see the WordPress > Appearance > Theme Editor option:
This can be due to a security plugin or this line of code in your wp-config.php
file, define( 'DISALLOW_FILE_EDIT', true );
this would be removed via FTP, or you can just make your theme changes via FTP and not use the “theme editor”
Best regards,
Mike