Update table columns without id in laravel

I want to create a page for admin where the page is only used to display data and update data. I create a table in the database without using an ID, because I don't want to create new data and only want to have one data other than that it can update that data.

this is my route

Route::group(['prefix' => 'admin', 'middleware' => ['web']], function () {
    Route::get('about', 'AboutController@index');
    Route::post('about/update', 'AboutController@update');
});

this is my model

public $timestamps = false;
protected $primaryKey = null;
public $incrementing = false;
protected $fillable = ['description'];

this is my controller AboutController.php

public function index()
{
    $data = About::first(); 
    return view('config_about.index', compact('data'));
}

public function update(Request $request)
{
    $this->validate($request, [
       'description' => 'required'
    ]);

    $data= About::first()->update($request->all());

    return redirect('admin/about');
}

and this is my form

{!! Form::open(['url' => 'admin/about/update', 'method' => 'post', 'class' => 'form-horizontal']) !!}
    <div class="form-group ">
        <div class="col-md-12">
            {!! Form::textarea('description', $data->description, ['class' => 'form-control']) !!}
            {!! $errors->first('description', '<p class="help-block">:message</p>') !!}
        </div>
    </div>

    <div class="form-group">
        <div class="col-md-2 col-md-offset-0">
            {!! Form::submit('Save', ['class' => 'btn btn-primary']) !!}
        </div>
    </div>
{!! Form::close() !!}

I hope the results can update the data but what I get is an error like This page isn’t working localhost is currently unable to handle this request. HTTP ERROR 500

via Chebli Mohamed