Mistake while paying advance tax

Hello Experts...

I am a salaried guy, I wanted to pay advance tax for year FY 2012-13 (assment year 2013-14).

When filling challan 280 on the website, I made a mistake. I selected Self assesment tax (code 300) and selected assessment year as 2012-13 and paid the money. The money is debited from my account.

My income tax for assesment year 2012-13 is already done (received Intimation-143) and refund also received.

So this can be seen as mistake in assessment year...

Please advise me on what should I do now...

1. Should I simply write a letter to them with a copy of the challan and previous year intimation-143 and request that the assessment year should be changed. How long will this take to reflect in form26AS..?

OR

2. Do I need to file for rectification...?

Any help and advice in this will be really appreciated.

Thanks much,

Amit