模型在get(）方法(Laravel，PHP）之后不保存

我正在使用get（）方法从数据库中检索模型。当我想使用保存按钮修改字段时，我收到该错误：调用未定义的方法 Illuminate''Database''Eloquent''Collection：：save（）;

这是我的代码：

public static function authenticate($token='randomToken'){
$u = User::where('token',$token)// $token)
    ->get(['token']);
if ($u->count()==1){//User is authenticated
    $u->token = User::getGUID();
    $u->save();
    return true;
}
else {
    return false;
}
}
public static function getGUID()
{
if (function_exists('com_create_guid')) {
    return com_create_guid();
} else {
    mt_srand((double)microtime() * 10000);//optional for php 4.2.0 and up.
    $charid = strtoupper(md5(uniqid(rand(), true)));
    $uuid = substr($charid, 0, 8)
            .substr($charid, 8, 4)
            .substr($charid, 12, 4)
            .substr($charid, 16, 4)
            .substr($charid, 20, 12);
    return $uuid;
}
}