Here is the code;

add_filter('template', 'serve_default_to_iesix');
add_filter('option_template', 'serve_default_to_iesix');
add_filter('option_stylesheet', 'serve_default_to_iesix');
function serve_default_to_iesix($theme) {
	if(strpos($_SERVER['HTTP_USER_AGENT'], 'MSIE 6') !== false)
		$theme = 'default';

	return $theme;
}

Insert this code to your Theme’s functions.php. With this code we will make Browser detection and we tell wordpress ‘Change theme to default if our visitors are use IE6!‘.

You can change ‘default’ to classic or any theme name. You have to edit this lines for make changes.

$theme = 'default';