How to create a new user name, password, authentication

If it is new user registration you can insert the values in database and if the user id already available you can update the values. use bellow code insert values

private void Insert()
{
SqlConnection cn=new SqlConnection("Your String);
SqlCommand cmd=new SqlCommand("Insert Into YourTable(UserName,Password,Authentication)Values('"+txtuname.text+"','"+txtpwd.Text
+"','"+combobox.SelectedItem.Text+"')",cn);
cn.Open();
int _records=cmd.ExecuteNonQuery();
cn.Close();
}

if it is update then change the command like update table set column=value like you can.