mistake in tax payable calculation in assessment order

I have received a assessment order for the AY 2011-12 in which department determined a demand which is not calculated appropatly I.e. I have received a refund of rs 433 whereas the order showing refund of 3490 also self assessment tax paid by me is not considered in assessment order and showing as refunded...now what should I do now....is there any rectification done in section 154 or anything else should I done...kindly suggest