How to upgrade existing project from Laravel 5.3 to Laravel 5.5?
It actually depends on your project size, since the estimated upgrade duration is 1-2 Hours, you might simply wish to copy almost all your files (one by one of course) to a newly installed Laravel 5.7, since it's the current latest version. Otherwise you should do it for every version by following the documentation.
https://laravel.com/docs/5.4/upgrade
https://laravel.com/docs/5.5/upgrade
https://laravel.com/docs/5.6/upgrade
https://laravel.com/docs/5.7/upgrade
I usually copy my files over to a new project if the project is small.Otherwise I simply follow the guide.