Rectify mistakes made in filing returns

Hi all Urgently need your advice and suggestions here. I made two mistakes while filing the ITR. One was that I under-calculated the tax to be paid since I missed taking one income bracket into account. Another mistake was that instead of filing the tax amount I paid in the 'Total tax paid' column, I entered the amount in the 'Tax Payable' column instead. Now, I have got a communication from the IT department asking me to pay the entire tax amount plus interest in another couple of weeks time. I do not know how I should let them know that I have already paid a portion of it and so the interest charged should only be for that portion of tax I missed paying? Your suggestions would be really helpful.