When updating a belongsTo relationship, you may use the associate method. This 
method will set the foreign key on the child model:

    $account = App\Account::find(10);
    $user->account()->associate($account);
    $user->save();

When removing a belongsTo relationship, you may use the dissociate method. This
method will set the relationship foreign key to null:

    $user->account()->dissociate();
    $user->save();