function myprefix_comment_form_args( $args ) {
$args['label_submit'] = 'Custom submit text';
return $args;
}
add_filter( 'comment_form_defaults', 'myprefix_comment_form_args' );
Snippets: Alter the WordPress Comments Submit Button Text
All PHP snippets should be added via a child theme's functions.php file or via the Code Snippets Plugin (or alternative plugin)