Reversal of wrongly claimed itc

We have claimed 3,50,000 IGST credit by mistake in july 17 return. Now we need to reverse in August18 return. we can reverse it through 3B of August 18 but what is the rate of interest and do we need to pay the amount reversed that is 3,50,0000 in cash or 3,50,0000 added to output tax liability cab be paid through ITC accumulated.Please help