This guide will help you though the process of converting a System Center 2012 Configuration Manager Evaluation, to a Full Product Version.

From the start menu navigate to All Programs -> Microsoft System Center 2012 -> Configuration Manager, and select Configuration Manager Setup.

Select Perform site maintenance or reset this Site, and click Next.

Select Convert from Evaluation to Full Product Version, and enter your 25-character product key, and click Next. Volume License Customers can find the product key here: https://www.microsoft.com/licensing/servicecenter/default.aspx
(AFAIK all volume license customers has the same key for all System Center 2012 products).

Accept the license terms, and you’re done…