Downgrading SQL Server 2012 to Lower Edition
You cannot directly change edition of SQL Server 2012 from Enterprise to Standard. See this page for the supported edition change scenarios.

There are a few different ways to go about doing this -- some recommend making copies of the system databases and overwriting them on the new instance (make sure you're at the exact same SP/CU level), or you could just set up a new environment from scratch. It really depends on how complex your environment is. I would definitely recommend testing the process in a separate environment if you can.

Also, before you start, go through your databases and check sys.dm_db_persisted_sku_features to make sure there aren't any Enterprise edition features in use.