Do you know what version of PHP is your website running?
Contacting your hosting company is usually the easiest way to find out how you can update your PHP version. They probably already have newer PHP versions available, they just need to update your account.
You will have to ask them to be updated to a currently supported PHP version. At this moment, it’s safe to run on PHP 7.0, but it doesn’t hurt to get upgraded to PHP 7.1 or higher already.