Advance tax not given credit - how to reclaim

Here is the situation:

1. For FY 2014-15 I paid 35K Advance Tax in two separate installments - 15K in September 2014 and 20K in March 2015. Both are reflected in my 26AS as Advanced Tax Credits.
2. While filing returns for some other reasons I entered details of the first Adv tax payment correct as Adv tax, but the second one I incorrectly marked as 'Self Assessment Tax'.
3. In the U/S 143(1) dated 24-12-2015, the 15K adv tax was correctly recognized, but the 20K was struck out and I got 0 as the Self Assessment Tax, giving me a total tax credit of only 15K, even though the total tax I had remitted to govt was 35K
4. As there was a net refund even after this mixup, I did not have to pay anything more, but for some unfortunate reason, I only realized this goof up now.

What is the right way to reclaim the credit given that my U/S 143(1) order is almost 2 years old now?

Thanks.