IF You are using Self hosted services. you have access to SSH.
You can install the PHP Version with this command
sudo apt-get install php
PHP will update/ install the default version available in Server repository, or if your server package don’t have the latest version available can install latest version available package and then install PHP.
To add a specific version add version number as suffix.
sudo apt-get install php7.1
Check the php Version
Managed Hosting/ cPanel
If your using Managed hosting Server, you may not have access to SSH, but most of hosting services providers like hostgator, bluehost. They will provide cPanel or similar framework to maintain the server.
From cPanel at PHP filed you can update/ Change the PHP Version.
If your hosting provider is not have any option to update the PHP Version, contact the hosting provider.