CI_Upload::do_upload()

do_upload([$field = 'userfile'])

Performs the upload based on the preferences you’ve set.

<form method="post" action="some_action" enctype="multipart/form-data" />

If you would like to set your own field name simply pass its value to the do_upload() method:

$field_name = "some_field_name";
$this->upload->do_upload($field_name);