You can refer to our documentation to learn about how to change between PHP versions: https://tribulant.com/docs/hosting-domains/11211/change-php-version/
Your website's code must match the current PHP version you're using. If your PHP version is too high, such as using version 7.3 while your website's theme and/or plugin is meant to be used with PHP 5.4, then your website will run into issues. Some issues can include error messages on your website or on certain pages or sections, blank pages, or your whole website not loading. Changing PHP versions can sometimes fix your website.
There's nothing to lose when you switch PHP versions. If you chose the wrong one, simply switch it back.
Always make sure to keep your themes and plugins up to date and to keep using the latest or the previous version of a PHP version. If you notice that your theme and/or plugin is no longer being updated, it might eventually run into issues. Attempt to contact the developer or business, but if they're no longer responsive, then consider switching to other updated themes and/or plugins.