I'm not familiar with Elastix. So we need additional information about packaging system. What is the version of Openfire shipped with it? Or maybe Openfire was installed separately? At Downloads page you can see 4 packages for linux: http://www.igniterealtime.org/downloads/index.jsp

CentOS is RedHat derivative as i know, so rpm should be used (in case that it was installed with rpm package). How to upgrade rpm version: http://www.rpm.org/max-rpm/ch-rpm-upgrade.html

With standalone tar.gz version upgrade procedure is a simple copy on top of older version (with some exceptions).

So, it could be a rpm installation. Not sure how to check this. You can try downloading rpm package and doing rpm update.

So, are you familiar with the console (not matters, linux, windows or else)? I havent used rpm based system for a very long, so i dont know what are the graphical interface options. So i would go with the console.

1) Downloading openfire-3.5.1-1.i386.rpm from here http://www.igniterealtime.org/downloads/index.jsp

2) Start console in Elastix an type rpm -U /path_to_the_downloaded_package/openfire-3.5.1-1.i386.rpm (as it's said in http://www.rpm.org/max-rpm/ch-rpm-upgrade.html)

This server doesnt have internet connection? Just download new rpm, i gave you the link. Or download it elsewhere and transfer to that server (cd, usb stick, etc.).

My linux box doesnt have a graphical interface, so i normally download new versions with wget in console:

wget http://www.igniterealtime.org/builds/openfire/openfire_#_#_#.tar.gz (on your case shgould be #.rpm; # means version numbers)

Linux console commands are often similar to DOS, but this is not the place to teach how to use linux, sorry. You should try to learn it yourself. I have already gave you a link about rpm commands.