Evaluation period has expired, SQL Server 2012

How to upgrade Evaluation edition or use Licence key to the SQL Server Services.

Run SQL Server 2012 Setup file as administrator and select maintenance link as shown below. 

Select Edition Upgrade; Edition Upgrade meaning you are actually Upgrading Evaluation edition to Enterprise edition by entering Licence key. Make sure you have Enterprise Edition KEY only other editions keys are not Accepted. 

And finally restart services and start the Management Studio.