And if you mean for the websites, Plesk doesn't provide PHP, Plesk just uses whatever PHP version is installed on your server. My guess is you're running a Linux distribution that comes with PHP 4.
You could:
1) install third-party PHP 5 packages (Atomic Rocket Turtle has some for RHEL/CentOS/Fedora), or
2) migrate to a distro that comes with PHP 5 (I can recommend CentOS 5), or
3) build PHP 5 packages for your distribution yourself (some skill is needed here).
Be warned that PHP 5.2.5 is currently not playing nice with Plesk 8.2.1.