prod_1

Hi Gurus, i have used process instruction category prod_1 for GR in pi sheets. my order qty is 5. first time i entered 2 qty in pi sheet and done the co54, develiverd qty appeared as 2 in order. i want to enter again the remaining 3 qty in pi sheet, but observed that delivered qty field is inactive. please advise how to update the same and is there any other way to send the proces instruction category again or manually like co57. please advise.