How to replace old file with new file in codeigniter?

Question

I am using unlink to remove old file with new file, but it doesn't work. Please help me, I'm a new programmer and really stuck with this.

here's my code

   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');
            $id_ajukan = $this->input->post('id_ajukan');
            $query = $this->db->get('ajukan_keluhan', array('id_ajukan' => $id_ajukan));
            $ambil = $query->row()->upload_spk;
            unlink('./assets/upload/' . $ambil);
        }
    } else {
        if ($_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('ajukan_keluhan', array('id_ajukan' => $id_ajukan));
                $ambil = $query->row()->upload_spk;
                unlink('./assets/upload/' . $ambil);
            }
        } else {
            $data['upload_spk'] = $this->input->post('lama');
        }
    }

and this is code in view

<div class='form-group row'> <label for='uploadsp' class='col-sm-2 col-form-label'>Upload SP/SPK</label> <div class='col-sm-10' align='left'> <input type='file' class='form-control col-md-10' cols='10' rows='5' name='upload_spk' / accept='.jpg, .png, .pdf/*'><input type='hidden'  id='lama'  name='lama'  value='<?= $kelola_tiket->upload_spk ?>'> </div> </div>