Joomla have announced that they will no longer be supporting Joomla 2.5 from the end of December 2014. That means that all current Joomla 2.5 sites will no longer receive new updates and bug fixes.
Your site will continue to function as long as it has not been hacked. Unfortunately there are hackers out there that are constantly looking at ways to hack into your site and abuse the site information and server powers. The longer you leave your site before updating the more chance you will have of becoming a victim of these hackers.
Most template developers already have versions of their templates that are compatible with Joomla 3.3.
Before upgrading we recommend you to take a complete backup of your website, create a mirror website to perform the upgrade on to make sure everything is working correctly and then replace your original site with the newly updated site.
How do I upgrade Joomla?
The first step in any migration process is planning. Joomla! 2.5 core extensions will upgrade with a “one-click update.” All third party extensions including template need to be addressed individually. We have resources available to help with your migration planning.
Joomla 2.5 to 3 is a minor migration (or a mini-migration)
A mini-migration means that the Joomla core extensions (Articles, Menus, Modules, Categories, Contacts, Weblinks, Newsfeeds, Search) can be upgraded with a one click update. All third party extensions including template need to be addressed individually.