public function update($id='')
{

      $session_data = $this->session->userdata('logged_in');
        $data['username'] = $session_data['username'];

        $id=$this->input->post('update');
        echo "<pre>";var_dump($id);
            $dc=$this->input->post('dc');
            if($dc=='c'){
            $amount=$this->input->post('credit1');
                        }
            else if ($dc=='d') {
            $amount=$this->input->post('debit');
                                }

$data=array(
      'date' =>$this->input->post('TDate'),
      'code' =>$this->input->post('TName'),
      'project' =>$this->input->post('TName1'),
      'part' =>$this->input->post('part1'),
      'part1' =>$this->input->post('part2'),
      'dc'=>$this->input->post('dc'),
      'amount'=>$amount,
   );
$this->db->where('recno', $id);
$this->db->update('daybook', $data);
$this->session->set_flashdata('Add1', 'Updated Successfully');
redirect('BookKeeping/daybook','refresh');
}

<a href="<?=site_url('BookKeeping/update/'.$currentID)?>" class="btn btn-info btn-"><i class="icon-new position-left">
                        <input type="hidden" name="update" id="id" value="<?php echo $currentID?>">
                    </i>Update</a>