If you need to upgrade SQL Express version to FULL you can proceed executing following command:
setup / ACTION = editionupgrade / INSTANCENAME = <INSTANCENAME> / PID = <XXXXX-XXXXX-XXXXX-XXXXX>
where <INSTANCENAME> is the name of the instance to be updated (default is SQLExpress) and <XXXXX- XXXXX- XXXXX- XXXXX- XXXXX> represents the license of the full version.
[original article http://www.achab.it/blog/index.cfm/2014/12/aggiornare-sql-server-express-a-una-versione-completa.htm]