I could be wrong, but I don't think Plesk 9 works with PHP 5.3, partly because of the reliance on the deprecated safe mode.
If you want to take the risk, and have sufficient backups, I believe the remi repository contains PHP 5.3. See here. If you end up doing a MySQL upgrade as well, make sure to do a full MySQL dump first, upgrade, then reimport the dump.
If you find PHP 5.2.14 would be suitable, you can use the atomic repositories:
wget -q -O - http://www.atomicorp.com/installers/atomic | sh
yum update php
You may find that php -v gives an error about ioncube. If it does, go into /etc/php.d/ioncube.ini (or similar) and change the 5.1 or 5_1 to 5.2 or 5_2 (I can't remember for sure). You may also need to remove php-sqlite2 before you can upgrade on a Plesk server, but you're unlikely to be using it (check dependencies).