Can't update primary key

here in your model - your update method

PHP Code:

// update data
    function update($id, $data)
    {    
    $this->db->update($this->table, $data);
    } 


you are passing $id and $data to update method...
but then you aren't using the $id
so then the database does not know which record you want to update

so you want a line in the method before the update line, like

PHP Code:

$this->db->where('nis', $id);