If you are using a trial version of SharePoint Server 2013 and want to upgrade to a licensed product edition, you can purchase a license and perform the upgrade on the SharePoint Central Administration itself. Below are the steps to upgrade from trial to licensed.
If you have a trial you will see the following options in Central Admin.
Just click it and type the serial key you just purchased and SharePoint will take care of the rest. The whole licensing process should take just a couple of minutes. Suffice to say the server must be connected to the internet.
by Edge Pereira