How to change Billing status of delivery-related billing documents?

Hi Experts,

Now I meet with the problem with the billing status.

I want to create a billing reference delivery notes, but the billing staus of delivery-related billing docuemnts is blank, I want to chage it to 'A' so as to create a billing refer to it.

Can anyone give me some hint to make it true?

Thanks a lot.

Best regards,

Chris Gu