wrongly entered exports under payment basis in other than zero rated column in 3b

Hi frnds,
Please help me to resolve the query.. In Dec 2018 gstr 3b return, instead of entering exports (under payment basis), in zero rated ourward supplies category, I have entered such exports under other than zero rated category. Due to that, refund has been delayed which should be credited to us. Now, in January 2019, may I enter the IGST(refund) amount which was not shown in Dec, under zero rated category? And is it possible to reduce the liability in January in non zero rated category as the same was inflated in Dec by including exports (under payment basis)?