How to change home or site url using action hooks or filter?

Ok. Then you can use template_redirect hook and wp_is_mobile() conditional tag. wp_is_mobile() will work on device only. If you resize the window then it will not work. Now you can try this kind of code in your functions.php file

add_action('template_redirect', 'redirecting_to_mobile_site');
function redirecting_to_mobile_site(){
  if( wp_is_mobile() ){
    wp_redirect( 'http://m.example.com', 301 ); exit;
  }
}