Hi, i have created a move order form .. now i want update serial number in a move order from backend

iam facing problem in move order:: i have created a move order form .. now i want update serial number in a move order from backend .. but its updating serial number and remaining column update.

if i give serial number is null in API its updating serial number as null..

declare

l_hdr_rec inv_move_order_pub.trohdr_rec_type := inv_move_order_pub.g_miss_trohdr_rec;

l_line_tbl inv_move_order_pub.trolin_tbl_type := inv_move_order_pub.g_miss_trolin_tbl;

x_return_status VARCHAR2 (100);

x_msg_count NUMBER;

x_msg_data VARCHAR2 (4000);

x_hdr_rec inv_move_order_pub.trohdr_rec_type := inv_move_order_pub.g_miss_trohdr_rec;