You are using upadate command.
SqlDataReader
work only with select command.
here you should use
cmd.ExecuteNonQuery();
SqlCommand cmd = new SqlCommand("update tbl_register set password='" + newpass + "',conpassword='" + newpass + "' where loginemailid='" + loginemailid + "' and password='"+ oldpass +"'", cn);
cmd.ExecuteNonQuery();
lblmsg.Text = "Record Updated";