F form

If Assesment had already been done by Sales Tax what should we do if we found one mismatch in Press printed F Form & want to modify it??

Actually by mistake one Transfer sale had not been showed in F Form ( Front & Back side) which is reflect in Balance sheet & due to absence of Hard copy of F Forms sales Tax charged Tax on the difference Value. But after assesment done it has been found & modify accordingly.

What is the procedure to modify it before sales tax???

Rajib