Roopesh,
If the mistake happened before 7 days, then it is better to approach the particular Branch of the Bank from which you made the payment. As per the new scheme of challan correction, even branch managers of banks are authorized to make changes in the OLTAS application. But for such a step, a request should be sent in writing to the Branch Manager informing him of the changes in the challan. Also the timeframe within which action has to be taken is 7 days from receipt of your letter.
Please send the following documents to the bank
1. Copy of your PAN card
2. Copy of the challan
3. Letter stating the changes required in the challan. (in Duplicate)
4. if assessee is other than an Individual, Original Authorisation (in Stamp paper)
I have attached the Format of the letter for the challan corrrection
After the the time window has expired, the Assessee can approach the AO