you can't modify GSTR-2A, it's a read only form.If the recipient finds that certain inward supplies made to him in the tax period are not reflected in the Form GSTR-2A, the recipient can manually add the details of such supplies in Form GSTR-2. Such additions will be communicated to the supplier in Form GSTR-1ANS. The supplier can accept/reject such modifications before 17th of the succeeding period, upon which, Form GSTR-1 filed by him would stand amended.