Additional Data 1 Field is unable to change in the sales order Header

Hi All,

  Iam Trying to change one field under the Tab Additional Data 1, and iam unable to change the Same. I have even checked in the Customer Master Data (sales are data) nothing is maintained under Additional Data ,

There are no preceding Documents to the sales order

Could any body guide us how to change any of the field, if i change Then i go to VA02, the old Vaiue in the Field i could see....

Thanks in advance

Regards

BR