First of all, Laravel 5.5 needs PHP version 7+. Open your composer.json file and change the version.

“require”: {
“php”: “>=7.0.0”,
“fideloper/proxy”: “~3.3”,
“laravel/framework”: “5.5.*”,
“laravel/tinker”: “~1.0”
}

Then Open terminal or cmd prompt and run command – composer update

If you need any further details, refer upgrade Guide documentation : https://laravel.com/docs/5.5/upgrade