Intro

The migration from Joomla 2.5 to 3.x is considered a mini-migration. This is because the Joomla core extensions will upgrade with a “one-click” upgrade via the Joomla! Update component in the backend administrator side of Joomla. Many third-party extensions are a one-click upgrade too. Some are not. You need to look at each one and determine what path the extension needs to follow to get from 2.5 to 3.x. If you haven't already, you might be interested in reading the Self Assessment and Planning for 2.5 to 3.x Migration prior to following the steps below.

Joomla Core Extensions:

Categories
Articles
Menus
Modules (core modules - not third-party)
Banners
Contacts
Messaging
Newsfeeds
Redirect
Search
Smart Search
Weblinks
For very large or complex 2.5 to 3.x migrations

read more at http://www.philmoresms.com/index.php/blog/88-blog/152-joomla-2-5-to-3-x-step-by-step-migration