Come utilizzare inserire o sostituire in codeigniter

Prova questo

public function save($data, $id)
{
    $query = $this->db->query("SELECT * FROM table_name WHERE id = '{$data['id']}' ");
    $result = $query->result_array();
    $count = count($result);

    if (empty($count)) {

        $this->db->insert('mytable', $data); 
    }
    elseif ($count == 1) {
        $this->db->where('id', $data['id']);
        $this->db->update('mytable', $data); 
    }
}