recommends that Joomla 2.5 users wait for Joomla 3.5 before migrating to the 3.x series.
You may see a notice in the Joomla Administrator about updating to version 3, but be very careful if you decide to update if you are still on version 2.5.

For those that would like to attempt to migrate before Joomla 3.5 there is this tutorial, but be forewarned that this is not an easy process and can be time consuming.

Always back up your Joomla files and database before attempting an update/migration.

SSL Secure Connection