To install the old version of Laravel, the easiest way is to use the composer create-project command. Using this command we only have to specify the project name and Laravel version that we want it to be installed and there you go it's ready.
composer create-project laravel/laravel your-project-name 6.0.*
If you prefer using the Laravel installer make sure to install it first by using the command below. And once it's installed do run "laravel new" command to create the latest version of Laravel.
composer global require laravel/installer laravel new your-project-name