From your cPanel, you can choose which version of PHP your website will use. This is done in the MultiPHP Manager. You can find more information on it here. However, updating the PHP version in your cPanel is only half of the battle.
You also need to ensure that your website is updated and compatible with newer versions of PHP. Every new PHP version comes with some new features and optimizations, but more importantly, new security updates. Keeping your website compatible with the latest PHP versions will help keep your site secure and lower your risk of downtime.
While most browsers are compatible with older versions of PHP, most CMS are not. If you are using a CMS such as WordPress, keeping it up-to-date should keep it compatible with the newest PHP versions. If you had someone build your site for you, you may need them to update your site from time to time to keep the site updated.
As older versions become obsolete, they start to become a security issue as they are no longer being patched. For this reason, we will, from time to time, remove older versions of PHP from our servers. If your site is still using an outdated version of PHP that has been removed, you will be updated automatically to the new default version. If your website is not compatible with this new version, your website will no longer function.
While we strive to never remove a PHP version with ample warning, there may be a time when a version of PHP becomes too large of a threat to leave on our servers. In this case, we may need to remove the version swiftly and without warning to protect your account and our servers. If your website was only compatible with that version, your site will not work until it is updated. This is another reason why you should keep your site updated.
When updating your PHP version, it is relatively safe to upgrade within an major release. For example, if you are upgrading from PHP 7.0 to PHP 7.4, you should be fine. However, if you are updating from PHP 5.2 to PHP 7.4, you are more likely to run into issues. It is recommended that you update to the last minor version of a release before updating to a newer major version. In the above example, you would want to update from PHP 5.2 to PHP 5.6 first. If you do not have any issues with PHP 5.6, next try to update to PHP 7.0 (there is no PHP 6). If still good, proceed to the newest PHP 7 version (7.4). If your site is not compatible with version 7.0, you will need to revert it back to PHP 5.6 and then update your website coding.