When Plesk is installed, it does not dictate the version of software installed, it simply uses the stable version that is installable from the base repository. This means that packages and daemons such as PHP or modules are not installable from inside the control panel.

To update or install packages you will need to use the standard package manager for your respective operating system.

CentOS/RHEL

You will use the yum package manager. http://linux.die.net/man/8/yum

Example:

yum update php
yum install php-mcrypt

Ubuntu/Debian

You will use the apt package manager. http://wiki.debian.org/apt-get

Example:

Apt-get install php

FreeBSD

You will use the Ports package manager. http://www.freebsd.org/ports/index.html

You will find the current ports installed on your server in /usr/ports
Once you have found the appropriate ports directory for your package you will need to make or compile the appropriate files:

Make
Make Install
Make clean