I am facing a problem with my hosting server where php 5.6 has been installed which can't be upgrade to php 7. But my project has been completed in laravel 5.5 which need php 7 to run. Is there any solution to switch laravel 5.5 to laravel 5.4
In fact there is no easy way to do that. If application is using Laravel 5.5 features some parts of it won't work and you might waste a lot of time to finding and fixing those issues.
In my opinion much cheaper way would be changing hosting and in fact it would me more reasonable to use PHP 7.x than 5.6 in 2017/2018. If you look at PHP versions you will see, that for 5.6 only security fixes are applied at the moment