Instructions per http://plone.org/documentation/manual/upgrade-guide/referencemanual-all-pages

County site:

Plone version overview
Plone 2.5.1,
CMF-1.6.2,
Zope (Zope 2.9.6-final, python 2.4.4, linux2),
Five 1.3.8,
Python 2.4.4 (#2, Apr 12 2007, 21:22:23) [GCC 4.1.2 (Ubuntu 4.1.2-0ubuntu4)],
PIL 1.1.6

As the howto specifies, confirm the process in a test environment before touching the live site.

V2.5.1 is available here http://plone.org/products/plone/releases/2.5.1

V 2.5.2

tested with Zope 2.8, 2.9

"Universal" Linux/BSD/UNIX installer (builds Python and Zope from source)
a 32M tarball http://prdownloads.sourceforge.net/plone/Plone-2.5.1-UnifiedInstaller.tgz?download
Product package (requires Zope 2.9.4+ or Zope 2.8.7+ already installed)
http://prdownloads.sourceforge.net/plone/Plone-2.5.1-final.tar.gz?download

V 3.2

tested with Zope 2.10

The latest stable release is 3.2
http://plone.org/products/plone/releases/3.2
3.2 Unified Installer - for Linux/BSD/OS X/UNIX/Solaris (compiles and installs Python, Zope and Plone plus dependencies for you)
http://launchpad.net/plone/3.2/3.2/+download/Plone-3.2-UnifiedInstaller.tgz