Settlement date in shipment cost

Dear Experts

i am preparing a shipement cost document wherein i have maintained the setlement date as "0004"-stage start actual.

The problem what i am facing is that the user wants to change this date at the time of transferring the cost to accounting but when he tries to open this shipment cost document in VI02 this field "settlement date" is greyed or freezed.

I wish to change this date according to my will.Kindly advise as to how this can be done.

Regards

Sanjeev