The changes you are trying to make is a customization work. Custom changes need to be made inside the theme file.
But if you are technically sound in wordpress and understand the theme files properly you can take the following reference or else it may mess up your theme;
1. Go to your index.php file and make changes.
2. You can simply add < ?php comment_form(); ?> inside index.php file within your particular blog style,
i.e just below the function get_template_part(). for instance;