BDC for Tax code updation

Hi all,

Iam trying to update tax code for open POs using T.Code ME22. For this iam writing BDC using call transaction.

I have PO, POitem,INTROW,EXTROW,Required Tax code in input file.

Pls guide me how to handle the updation of tax code with the available information.

Note: All the POs are last fiscal yrs open POs.