Correcting assessment year in self-assessment challan

Hi,

I received the Income tax Intimation U/S 143(1) for AY 2012-13 in my email. I had filed ITR electronically. It contains a demand for a significant amount. On comparing 'As Provided By Tax Payer in Return of Income' with 'As Computed Under Section 143(1)' columns, I found that the descrepency lies in the self assessment tax field. None of the self-assessment payments are being considered and also being listed in the "Unmatched Tax Payment Claims" section.

I logged onto my bank's website through which I had paid the self-assessment tax and I think I have found the mistake - I specified the assessment year as 2011-12. Also, I confirmed the same by checking Form 26AS for AY 2011-12. It shows the CINS there correctly. The CINS were meant for AY 2012-13 !!!

How do I go about rectifying this error?

Thanks,
Rohit