Rectification

I am Supriya (94 Points)

14 May 2019  
For A.Y.18-19, there was LTCG from MF (equity) which was taxed instead of showing it as exempt income.
The ITR has been processed, 143(1) has been received and refund (wrong amount i.e. after considering LTCG as taxable) has been credited to bank.

Now the mistake has come to the notice of the assessee. The return cannot be revised as due date for revising was 31st March 2019.

What else can be done to rectify this error and get the balance/correct refund?