$(document).ready(function(){
  $('.fa-caret-down').click(toggle_menu_items);
});

function toggle_menu_items(e){
  e.preventDefault();
  submenu = $(e.currentTarget).next();
  $(submenu).slideToggle(500, refresh_menu_items);
}

function refresh_menu_items(e){
  if( $(submenu).is(":visible")){
    $(e.currentTarget).removeClass('fa-caret-left');
    $(e.currentTarget).addClass('fa-caret-down');
  }
  else{
    $(e.currentTarget).removeClass('fa-caret-down');
    $(e.currentTarget).addClass('fa-caret-left');
  }
}