How to Update PO(ekko and ekpo) by using Bapi

Hi Experts,

I am very new in Abap ,I want to update some fields of Purchase order through Bapi.

In  Selection screen  i need these things,
PO Number ,  material, Delivery date

and i want to update the below fields of  Purchase order(PO)

1.   Street/House number  ,Postal Code/City  in header level (Give any dummy values for street/house number city  and postal code)
2.  PO quantity, Delivery date , net price  for all items which has matching material as in selection screen

and i am not giving data from excel sheet ,,i am giving data directly or through selection screen for the above mentioned fields.

please give me some solution or sample code so that i can do it.

Thankx in advance..

Ritesh