@blackrose The best way to do this is by changing the value of the Laravel version inside the composer.json file to "laravel/framework": "5.5.*",. delete vendor folder and run composer install
I guess for downgrading you can just follow the same steps in reverse.
Really though you are better off upgrading php. There should not be any issues if you stay within the 7.1 branch and Laravel 5.6 works great in 7.2 so upgrading should be an easy process unless you have some older applications on the server as well. PHP should be kept up to date within the minor branch or you will miss out on any security fixes in each release.