What's new in SharePoint 2013 upgrade

SharePoint 2013 does not support in-place upgrade for an existing environment. You must use the database-attach upgrade method to upgrade your databases to a new environment that is based on SharePoint 2013. Also, to provide more flexibility to farm administrators and site administrators, the upgrade process has changed to separate upgrade of the software and databases from upgrade of the sites.

The database-attach upgrade method offers more flexibility, more control, and a better success rate. To use a database attach upgrade, you complete the following tasks:

1. Create and configure a new farm that is separate from the old farm
   
2. Copy the content and services databases to the new farm
   
3. Upgrade the data and sites
   
   You can upgrade the content databases in any order and upgrade several databases at the same time to speed up the overall process

http://technet.microsoft.com/en-us/library/ee617150.aspx