Upgraded Laravel 5.6 project has old Bootstrap

I'm new in programming world and I've some problem.

I have project created in Laravel 5.5. I upgraded it to 5.6 by edit composer.json and execute composer update.

I observed some problems with Bootstrap features, so I tried to check my bootstrap version, and in header of public > css > app.css I saw * Bootstrap v3.3.7

How to upgrade it?

My tests: 1) composer require twbs/bootstrap:4.0.0 composer update It doesn't help. 2) New project has the newest Bootstrap version.

via Chebli Mohamed