[eluser]Glazz[/eluser]
Hmm i still don't understand very well, your english is not the best..
You should update a row on the database using his unique id.
Something like this:
Code:
$params = array(
'cat_name' => $this->input->post('cat_name')
);
$this->db->where('cat_id', $your_cat_id_here)->update('categories_table_name', $params);
Where you have $your_cat_id_here it is the category unique id on the database.