If you have a Joomla 2.5 website, you are on the right path to the Joomla 3 series. However, don’t just click any upgrade button in the administrator panel because there is more to upgrading than you see.
When you click the upgrade button, what happens behind the scenes are updates to the core Joomla database. Before updating to Joomla 3, you will need to have a Joomla 3 ready template for your site as well. Joomla 2.5 templates don’t work with Joomla 3. If you’re using a commercial template, check with the developer to see if there is a Joomla 3 version.