Modifying the code a little seemed to resolve the issue. Here is the new code:
private void ChangeMajor_Button_Click(object sender, RoutedEventArgs e)
{
var query = Roster_Students.Where(s => s.Tno == Tno_TextBox.Text);
foreach (var student in query) {
student.Major = ChangeMajor_TextBox.Text;
App.DBConnection.Update(student);
}
}