In Server Explorer, choose the Connect to Database button.
In the Add Connection dialog box, specify the following information:
Data Source: Microsoft SQL Server (SqlClient)
Server Name: (LocalDB)\v11.0
Attach a database file: Path, where Path is the physical path of the primary .mdf file.
Logical Name: Name, where Name is the name that you want to use with the file.
Choose the OK button.
When prompted, choose the Yes button to upgrade the file.
The database is upgraded, attached to the LocalDB database engine, and no longer compatible with SQL Server 2008 Express.