This guide will help you though the process of converting a System Center 2012 Configuration Manager (SCCM) 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.

Convert System Center 2012 from Evaluation to Full Product Version

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).

Convert System Center 2012 from Evaluation to Full Product Version

Accept the license terms, and you’re done…

Convert System Center 2012 from Evaluation to Full Product Version

Enjoy.