If you have a network licensed software, you will just have to install the 2013 software as a Network/Network deployment, then point the computer to the server where the license file is. The license file generated under the current network version allows you to access the current product version and up to 3 versions down of your products.
On the other hand, if you have a standalone license and are on active subscription, you will need to submit a previous version request via Subscription Center to obtain a serial number and product key for the 2013 version.
Kindly refer to the previous version FAQs for more information:
http://usa.autodesk.com/adsk/servlet/ps/dl/item?siteID=123112&id=22128897&linkID=12305695