//Untuk di Controller
public function update_password()
{
    $this->load->model('m_auth');

    $user = $this->session->userdata('username');
    $opass = $this->input->post('old_password');
    $npass = $this->input->post('new_password');
    $rpass = $this->input->post('repeat_password');

    $cek = $this->m_auth->cek_username($user,'user');

    if($rpass == $npass){
        if($cek->num_rows() > 0){
            $row = $cek->row();
            if($opass == $row->password){
                $data = array('password' => md5($npass));
                $this->m_auth->ganti_password($user,$data,'user');
                redirect('lokasi redirect setelah berhasil ganti password');
            }else{

            }
        }else{

        }
    }else{

    }
}

//Untuk Di Model
function cek_user($user,$table){
    $this->db->where('username',$table);
    return $this->db->get($table);
}

function ganti_password($user,$data,$table){
    $this->db->where('usermae',$user);
    $this->db->update($table,$data);
}