This is a major issue with GST return that it can not be revised once you filed the same. There are so many taxpayers who are facing this problem. There is a way to correct this mistake.You can adjust outward supply in upcoming months. For example your outward supply was Rs. 30 K for the month of Aprilu201918 and you have mentioned only Rs. 20 K by mistake. There is a difference of Rs. 10 K and this amount can be adjusted in next month Return.You can refer GST Circular No. 26/26/2022 GST for more information in this regard.Hope it helps.