Laravel Editing post doesn't work

There are routes

Route::get('posts', 'PostsController@index');
Route::get('posts/create', 'PostsController@create');
Route::get('posts/{id}', 'PostsController@show')->name('posts.show');
Route::get('get-random-post', 'PostsController@getRandomPost');
Route::post('posts', 'PostsController@store');
Route::post('publish', 'PostsController@publish');
Route::post('unpublish', 'PostsController@unpublish');
Route::post('delete', 'PostsController@delete');
Route::post('restore', 'PostsController@restore');
Route::post('change-rating', 'PostsController@changeRating');

Route::get('dashboard/posts/{id}/edit', 'PostsController@edit');
Route::put('dashboard/posts/{id}', 'PostsController@update');

Route::get('dashboard', 'DashboardController@index');
Route::get('dashboard/posts/{id}', 'DashboardController@show');
Route::get('dashboard/published', 'DashboardController@published');
Route::get('dashboard/deleted', 'DashboardController@deleted');

methods in PostsController

public function edit($id) 
{
    $post = Post::findOrFail($id);  
    return view('dashboard.edit', compact('post'));
}

public function update($id, PostRequest $request) 
{
    $post = Post::findOrFail($id);
    $post->update($request->all());
    return redirect()->route('dashboard.show', ["id" => $post->id]);
}

but when I change post and click submit button, I get an error

MethodNotAllowedHttpException in RouteCollection.php line 233:

What's wrong? How to fix it?

via Heidel