Entity Framework related entities on object is not updated

Question

Whenever I try to change Department and Position objects of the Employee class they are not updating inside database on the Employee entity. For example, I want to change employee Position from junior (id - 1, name of the position - junior) to middle (id - 2, name of the position - middle). But, whenever I try to save this changes to the database they don't apply.

My Employee entity

    public class Employee
{
    public int EmployeeId { get; set; }
    public string FullName { get; set; }
    public Department Department { get; set; }
    public EmployeePosition Position { get; set; }
    public string Head { get; set; }
    public double Salary { get; set; }

    public Employee()
    {
        Department = new Department();
        Position = new EmployeePosition();
    }
}

EmployeePosition entity

    public class EmployeePosition
{
    public int Id { get; set; }
    public string Name { get; set; }
}

My modify employee method in ViewModel

        public void ModifyEmployee(int id)
    {
        Employee employee = employeeRepository.GetEmployee(id);
        using (var context = new PowerCoEntity())
        {
            employee.Position = context.EmployeePositions.FirstOrDefault(d => d.Id == SelectedPositionId);
            employee.Department = context.Deprtments.FirstOrDefault(d => d.DepartmentId == SelectedDepartmentId);
            if (SelectedHeadId != null)
                employee.Head = employeeRepository.GetHeadName(SelectedHeadId.Value);
            else employee.Head = "";
        }

        employee.Salary = Employee.Salary;
        employee.FullName = Employee.FullName;

        employeeRepository.ModifyEmployee(employee);
    }

Modify method in the context class

 public void ModifyEmployee(Employee employee)
    {
        using (var context = new PowerCoEntity())
        {
            context.Entry(employee).State = EntityState.Modified;
            context.SaveChanges();
        }
    }

UPDATE 1

I got it working by changing ModifyEmployee methods in the ViewModel and Context class. But I need to pass all the parameters through method now. How can I be able to use different contexts to modify object? Context class:

public void ModifyEmployee(int id, int selectedPositionId, int selectedDepartmentId, int? selectedHeadId, Employee employee)
    {
        using (var context = new PowerCoEntity())
        {
            Employee emp = context.Employees.FirstOrDefault(e=> e.EmployeeId == 32);
            emp.Salary = employee.Salary;
            emp.FullName = employee.FullName;
            emp.Department = employee.Department;
            emp.Position = context.EmployeePositions.FirstOrDefault(d => d.Id == selectedPositionId);
            emp.Department = context.Deprtments.FirstOrDefault(d => d.DepartmentId == selectedDepartmentId);
            if (selectedHeadId != null)
                emp.Head = GetHeadName(selectedHeadId.Value);
            else emp.Head = "";

            emp.Salary = employee.Salary;
            emp.FullName = employee.FullName;
            context.Entry(emp).State = EntityState.Modified;
            context.SaveChanges();
        }
    }

ViewModel:

        public void ModifyEmployee(int id)
    {
        Employee employee = new Employee();

        employee.Salary = Employee.Salary;
        employee.FullName = Employee.FullName;

        employeeRepository.ModifyEmployee(id, SelectedPositionId, SelectedDepartmentId, SelectedHeadId, employee);
    }