you should upgrade the platform (please note system requirements), then install the "installer/upgrader" app that you download from the Splunk apps website. Run it, follow its advice, and it will perform the upgrade for your search head. You will then need to deploy the updated Technology Add-ons where necessary.
This feels kind of like asking someone to explain thermodynamics via Twitter... It's a major release of our security platform which allows you to take advantage of the features in a major release of our platform. There are many improvements, a summary of which can be found here: http://docs.splunk.com/Documentation/ES/3.0/RN/Enhancements
I think that babyd was interested in suggestions and best practise to adopt planning a migration from ES 2.x to ES3. We're also interested: some customer with ES 2.4 are asking for migration pros and cons and we need more accurate infos to plan the migration and estimate possible costs and efforts.