You can change to a different PHP version by going to your control panel, and then choosing the domain name you wish to change settings for (if you have more than one in your account), and then into the “Advanced” settings for the web hosting. The option to change your PHP version is found in there.
Before updating your PHP version to a newer version, you should make sure that all your components (e.g. plugins and themes) are up-to-date. Old components may not be compatible with newer PHP versions that did not exist at the time they were last updated. If your site does not work after updating version, then this is likely to be the cause, and you may need to switch back to the previous version to rectify that.
Posted in: Questions about PHP scripting