Updating laravel to laravel 5.5 from laravel 5.4
as stated in the error message Laravel 5.5 is still in dev, meaning there's no stable release and by default Composer limits you to downloading only stable numbered releases to avoid you downloading any packages that are in active development and may contain breaking changes.
You can circumvent this by adding the following two lines to your composer.json file
"minimum-stability": "dev","prefer-stable": true,This way it prefers stable releases wherever it can find them but it will allow you to download dev packages if nothing else is available.
I think you should update the PHP version as well. If you check the official doc, it says the minimum PHP requirement 7.0.0.