For Software Generation 7 SmartVPSs you'll see the option to change the PHP version via our Dashboard. Login then click VPSs -> Manage -> Web Sites. Find the site or subdomain and then click the PHP Version: X.X (change) link. Version changes are reflected immediately.
For all other Software versions we can upgrade or downgrade your SmartVPS to nearly any version of PHP and MySQL that you like. Just send us an email to email@example.com and we'll get it done ASAP.