$(document).ready(function(){
  $('.fa-caret-down').click(function(e){
    e.preventDefault();
    submenu = $(e.currentTarget).next();
    $(submenu).slideToggle(500, toggle_menu_items(e));
  });
});

function toggle_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');
  }
}