GSTR-1 Rectification

For the month of August GSTR-1 has been filed but in taxable amount freight recovered not included by mistake but tax amount & invoice amt correctly entered. For example in an invoice we have charged 90 for goods supplied & 10 for freight charges but on GST portal we have showed 90 taxable value but tax has been correctly shown of 18 and also invoice value is also correctly entered 118. So how can i amend only taxable value in GSTR-1.

Please help & suggest.