Cara replace upload file lama dengan file baru gmn ya?

if ($data['upload_spk'] = "") {
            $config['upload_path']  = './assets/upload';
            $config['allowed_types'] = 'jpg|png|pdf';

            $this->load->library('upload',  $config);
            if (!$this->upload->do_upload('upload_spk')) {
                echo "upload gagal";
                die();
            } else {
                $data['upload_spk'] = $this->upload->data('file_name');
            }
        } else {
            if ($_FILES['upload_spk']['name'] != "") {
                $config['upload_path']  = './assets/upload';
                $config['allowed_types'] = 'jpg|png|pdf';

                $this->load->library('upload',  $config);
                if (!$this->upload->do_upload('upload_spk')) {
                    echo "upload gagal";
                    die();
                } else {
                    //upload_spk diubah
                    $data['upload_spk'] = $this->upload->data('file_name');
                    $id_ajukan = $this->input->post('id_ajukan');
                    $query = $this->db->get('pengajuan_lelang', array('id_ajukan' => $id_ajukan));
                    $ambil = $query->row()->upload_spk;
                    unlink('./assets/upload/' . $ambil);
                }
            } else {
                $data['upload_spk'] = $this->input->post('lama');

            }
        }