C# change entity framework connectionstring dynamically

public class StudentController : ApiController
{
    public IHttpActionResult GetAllStudents ()
    {
        IList<StudentViewModel> students = null;

        using (var ctx = new SchoolDBEntities())
        {
            students = ctx.Students.Include("StudentAddress")
                        .Select(s => new StudentViewModel()
                        {
                            Id = s.StudentID,
                            FirstName = s.FirstName,
                            LastName = s.LastName
                        }).ToList<StudentViewModel>();
        }

        if (students.Count == 0)
        {
            return NotFound();
        }

        return Ok(students);
    }
}

string connectionString = new System.Configuration.ConfigurationSettings.AppSettings["ConnectionString"]);

System.Data.SqlClient.SqlConnectionStringBuilder scsb = new System.Data.SqlClient.SqlConnectionStringBuilder(connectionString);

EntityConnectionStringBuilder ecb = new EntityConnectionStringBuilder();
ecb.Metadata = "res://*/Sample.csdl|res://*/Sample.ssdl|res://*/Sample.msl";
ecb.Provider = "System.Data.SqlClient";
ecb.ProviderConnectionString = scsb.ConnectionString;

dataContext = new SampleEntities(ecb.ConnectionString);

public class SingleConnection
    {
        private SingleConnection() { }
        private static SingleConnection _ConsString = null;
        private String _String = null;

        public static string ConString
        {
            get
            {
                if (_ConsString == null)
                {
                    _ConsString = new SingleConnection { _String = SingleConnection.Connect() };
                    return _ConsString._String;
                }
                else
                    return _ConsString._String;
            }
        }

        public static string Connect()
        {
            //Build an SQL connection string
            SqlConnectionStringBuilder sqlString = new SqlConnectionStringBuilder()
            {
                DataSource = "SQLSERVER\\DEV".ToString(), // Server name
                InitialCatalog = "TestDB",  			  //Database
                UserID = "devuser",         			  //Username
                Password = "devpassword",  				  //Password
            };
            //Build an Entity Framework connection string
            EntityConnectionStringBuilder entityString = new EntityConnectionStringBuilder()
            {
                Provider = "System.Data.SqlClient",
                Metadata = "res://*/DevModel.csdl|res://*/DevModel.ssdl|res://*/DevModel.msl",
                ProviderConnectionString = @"data source=SQLSERVER\DEV;initial catalog=TestDB;user id=devuser;password=devpassword"   // sqlString.ToString()
            };
            return entityString.ConnectionString;
        }
	}