How to set config file value in Laravel?

config()->set('app.url','http://localhost:8000');

//OR onther way

Config::set('app.url','http://localhost:8001');

Example :

return [
    'facebook' => [
        'url' => 'https://www.facebook.com/your_profile_name',
        'username' => 'your_profile_name'
    ],
    'twitter' => [
        'url' => 'https://twitter.com/web_com',
        'username' => 'web_com'
    ]
];